A lot of things pre-date capitalism, but slavery in colonies wasn't an accidental or inessential result if we take note of the colony economic theorist, E.G. Wakefield noting how without the established infrastructure and population, a capitalist mode of production was infeasible.
See 'Plans of Immigration', in E.G. Wakefield's Letter from Sydney
made note of his own discovery of how colonies required more forceful means than the coercion established from primitive accumulation.
Political economy confuses on principle two very different kinds of private property, of which one rests on the producers’ own labour, the other on the employment of the labour of others. It forgets that the latter not only is the direct antithesis of the former, but absolutely grows on its tomb only. In Western Europe, the home of Political Economy, the process of primitive accumulation is more of less accomplished. Here the capitalist regime has either directly conquered the whole domain of national production, or, where economic conditions are less developed, it, at least, indirectly controls those strata of society which, though belonging to the antiquated mode of production, continue to exist side by side with it in gradual decay. To this ready-made world of capital, the political economist applies the notions of law and of property inherited from a pre-capitalistic world with all the more anxious zeal and all the greater unction, the more loudly the facts cry out in the face of his ideology.
It is otherwise in the colonies. There the capitalist regime everywhere comes into collision with the resistance of the producer, who, as owner of his own conditions of labour, employs that labour to enrich himself, instead of the capitalist. The contradiction of these two diametrically opposed economic systems, manifests itself here practically in a struggle between them. Where the capitalist has at his back the power of the mother-country, he tries to clear out of his way by force the modes of production and appropriation based on the independent labour of the producer.
The same interest, which compels the sycophant of capital, the political economist, in the mother-country, to proclaim the theoretical identity of the capitalist mode of production with its contrary, that same interest compels him in the colonies to make a clean breast of it, and to proclaim aloud the antagonism of the two modes of production. To this end, he proves how the development of the social productive power of labour, co-operation, division of labour, use of machinery on a large scale, &c., are impossible without the expropriation of the labourers, and the corresponding transformation of their means of production into capital. In the interest of the so-called national wealth, he seeks for artificial means to ensure the poverty of the people. Here his apologetic armor crumbles off, bit by bit, like rotten touchwood.
It is the great merit of E.G. Wakefield to have discovered, not anything new about the Colonies , but to have discovered in the Colonies the truth as to the conditions of capitalist production in the mother country. As the system of protection at its origin  attempted to manufacture capitalists artificially in the mother-country, so Wakefield’s colonisation theory, which England tried for a time to enforce by Acts of Parliament, attempted to effect the manufacture of wage-workers in the Colonies. This he calls “systematic colonisation.”
First of all, Wakefield discovered that in the Colonies, property in money, means of subsistence, machines, and other means of production, does not as yet stamp a man as a capitalist if there be wanting the correlative — the wage-worker, the other man who is compelled to sell himself of his own free will. He discovered that capital is not a thing, but a social relation between persons, established by the instrumentality of things.  Mr. Peel, he moans, took with him from England to Swan River, West Australia, means of subsistence and of production to the amount of £50,000. Mr. Peel had the foresight to bring with him, besides, 300 persons of the working class, men, women, and children. Once arrived at his destination, “Mr. Peel was left without a servant to make his bed or fetch him water from the river.”  Unhappy Mr. Peel who provided for everything except the export of English modes of production to Swan River!
For the understanding of the following discoveries of Wakefield, two preliminary remarks: We know that the means of production and subsistence, while they remain the property of the immediate producer, are not capital. They become capital only under circumstances in which they serve at the same time as means of exploitation and subjection of the labourer. But this capitalist soul of theirs is so intimately wedded, in the head of the political economist, to their material substance, that he christens them capital under all circumstances, even when they are its exact opposite.
But once further developed slavery did become redundent and such was felt by the masses who lived in parts of the country where such production was already essential obsolete.https://www.marxists.org/archive/ilyenkov/works/articles/humanism-science.htm
This is precisely how Frederick Engels depicted the relationship between “scientific accuracy” and “the moral self-consciousness of the mass.” He remarked that science could not rely upon arguments derived from morality nor base its propositions on arguments derived from “moral sentiment.”
“Marx, therefore, never based his communist demands upon this. ... But what formally may be economically incorrect, may all the same be correct from the point of view of world history. If the moral consciousness of the mass declares an economic fact to be unjust, as it has done in the case of slavery or serf labour, that is a proof that the fact itself has been outlived, that other economic facts have made their appearance, owing to which the former has become unbearable and untenable. Therefore, a very true economic content may be concealed behind the formal economic incorrectness."(Engels, 1885 Preface to The Poverty of Philosophy)
The “moral sentiment of the mass” turns out to be correct in its stance against “strict science” which has not yet succeeded in getting to the heart of the matter precisely because the masses are truly caught in the vice of the contradiction between two categories of stubborn facts. In other words, the “moral sentiment” - humanistically-oriented consciousness - expresses in the given instance the presence of a real problem which must be resolved both theoretically and practically, the existence of an actual social contradiction, an outlet from which must be sought in a scientific manner.
Although this game of body counts won't amount to much other than morbid fascination.
In part, because such violence and death is abstracted entirely from the conditions in which they occurred and thus given significance.
Made so abstract is like family conflict scales that measure every incidence of violence as if all kinds of violence are considered morally equal by all persons.
I could speak of death tolls in a war but leave implied whose deaths were the 'bad' deaths and the others were a necessary sacrifice of the conflict.
To which every ruling class readily spills the blood of it's people.https://www.marxists.org/archive/connolly/1915/11/conscrpt.htm
Indeed that lesson has been all too tardily learned by the people and their leaders. One great source of the strength of the ruling class has ever been their willingness to kill in defence of their power and privileges. Let their power be once attacked either by foreign foes, or domestic revolutionists, and at once we see the rulers prepared to kill, and kill, and kill. The readiness of the ruling class to order killing, the small value the ruling class has ever set upon human life, is in marked contrast to the reluctance of all revolutionists to shed blood.
The French Reign of Terror is spoken of with horror and execration by the people who talk in joyful praise about the mad adventure of the Dardanelles. And yet in any one day of battle at the Dardanelles there were more lives lost than in all the nine months of the Reign of Terror.
Should the day ever come when revolutionary leaders are prepared to sacrifice the lives of those under them as recklessly as the ruling class do in every war, there will not be a throne or despotic government left in the world. Our rulers reign by virtue of their readiness to destroy human life in order to reign; their reign will end on the day their discontented subjects care as little for the destruction of human life as they do.
But in discussing the morality of violence requires an assessment of means and ends, where a violent means can and does often become justifiable even to the moast pacifist buddhist, one often frames violence as a necesasry means of self-defense in an erupting conflict.
But violence should attempt to be avoided, but its avoidance shouldn't be a metaphysical principle either, the issue is the balance of means and ends as appropriate to the circumstance.https://www.marxists.org/archive/ilyenkov/works/articles/humanism-science.htm
The real and very difficult problem, calling for a clear theoretical solution lies elsewhere. Is it admissible to interpret the formula: “that which serves the victory of communism is moral” to mean that in the name of this great cause “all is permitted,” that there are and can be no restrictions of a moral nature imposed here? Or might it be argued that even here not all is “permitted"?
Is there in general a limit beyond which a deviation, forced by extreme circumstances, from the abstract general norms of humaneness in the name of and for the sake of the triumph of a concretely and historically understood humanism is transformed into - in full agreement with the laws of the dialectic - a crime against the very goal for the sake of which the act was undertaken? To speak more to the point, can this fatal limit be determined, for it always exists somewhere or the other? In actuality this border forms the great divide between the authentic communism of Marx, Engels and Lenin and those “left” doctrines which interpret Marxist moral formula as indicating that “all is permitted.” It is one matter to understand that violence and murder are inevitable actions summoned by the extreme circumstances accompanying the deadly battle of the classes, actions to which the revolutionary must resort, recognizing fully their inhumanity. It is quite another matter, to look upon these activities as the optimal, the safest and even the only methods of establishing “happiness” on Earth. Both Marx and Lenin morally approved violence only in the most extreme circumstances, and then, only on the minimal scale, that which is absolutely necessary.
Lenin wrote that Communists are opposed to violence against people in general and they resort to coercion only when it is imposed upon them by authentic admirers of violence. The only justification for violence is as a means of opposing violence, as violence against the violent, but not as a means of influencing the will of the majority of the working people.
Therefore Communists are never the initiators of actions such as war or the “export of revolution” at the point of the bayonet. Lenin always categorically and consistently opposed “left” ideas of this type. In his understanding the scientific spirit of communism is always inseparably connected with the principle of humaneness in the direct sense of the word.
This also forms the principal difference between Lenin and those doctrinaires who allow themselves the pleasure of cynically counting up the number of human lives “worth” paying for the victory of world communism. ... As a rule such calculations in today’s world are the occupation of people characterized by primitivity both in terms of theory and in their moral profile. ‘
And when we look at the forced labour of the USSR, and the brutality, we should perhaps be attempting to look at it's relation to industrialization which is brutal at every point in time. Russia being such a backwater and underdeveloped nation, and the push to develop the country's infrastructure.
And they sought to industrialize at an extreme pace.https://www.nationalarchives.gov.uk/education/heroesvillains/g4/cs1/
The race to industrialise was spurred on by the fear that capitalist countries would try to destroy communism in the USSR. At the First Conference of Workers in 1931, Stalin delivered a passionate speech, commanding workers to play a crucial role in industrialisation.
He said: "We are fifty or a hundred years behind the advanced countries. We must make up this gap in ten years. Either we do it or they will crush us."
History can seem one of necessity, events only happening as they did, but this isn't the case, but we can certainly make intelligible some things when we examine the conditions rather than a pithy, this is bad and thus by implicit necessity so is this.
But this would require elaboration on ideas about how man makes his history in inherited circumstances.
But this is in part why I don't think one can really effectively negate the appeal of either, as the history of both is necessarily bloodied with revolutions, rapid development of production, even throw in for the USSR emerging from civil war to a world war which became a war for their very existence against the German war machine. Where can even see the rhetoric shift from that of socialist appeals to nationalist ones as the nature of the war was hardly one of aspiration for the future society but to even have a Russian society not slaughtered.
There seems to be the tendency for people to simply do terrible things then ask for forgiveness later.
We can't look at the US as it exists today without understanding the global economy and slave trade where sugar and tobacco were in high demand and the basis for its economic development. https://www.nytimes.com/interactive/2019/08/14/magazine/sugar-slave-trade-slavery.html
I think the point to make is whether one can propose a way of life that advances the overall human condition.
Leftists certainly see capitalism as a progressive development upon feudalism even in spite of the immense suffering it causes. Where many would like to generalize the middle class experience as opposed to the masses.https://www.marxists.org/archive/lenin/works/1916/miliprog/index.htm
The bourgeoisie makes it its business to promote trusts, drive women and children into the factories, subject them to corruption and suffering, condemn them to extreme poverty. We do not demand such development, we do not support it. We fight it. But how do we fight? We explain that trusts and the employment of women in industry are progressive. We do not want a return to the handicraft system, pre-monopoly capitalism, domestic drudgery for women. Forward through the trusts, etc., and beyond them to socialism!
For leftists, the attempt to establish socialism would be a development, although it is noted and was even predicted without certain conditions that the USSR would fail in it's goal without socialist revolutions through Europe. http://burawoy.berkeley.edu/Marxism/Marxism%20As%20Science.pdf
the defeat of the working class in the West. In Results and Prospects (1969) he wrote that failing a revolution in the West the Russian revolution would be aborted and would turn inward on itself. He anticipated the broad outlines of what actually happened after 1917. The tragedy of Trotsky's life was that he was destined to be the agent and the victim of his own accurate predictions -the involution of a Russian Revolution that was not followed by revolution in the West, the process he analyzed with great acuity in Revolution Betrayed ([I9361 1972).
And many of the appeals to anti-socialism register little more than cold war rhetoric than analysis.
It doesn't even get to the essential point of why would oppose it, appeals only to the risk of failure whilst diminishing the problems which give life to it as a proposed solution. For this reason such ideas will never simply die away as the same essential conflict exists and as such, the once radical quality of liberalism has destroyed even many of its progressive features to defend itself as its revolutionary tools became a weapon against themselves.https://www.marxists.org/archive/ilyenkov/works/articles/marxist-leninist.htm
The question for Marx arose in the following form: Is it possible (and if so, precisely how) to resolve the conflicts in the development of private property in the soil of that private property itself? “Peacefully?” This again is not the position of a communist. But it is the position of a theoretician and it retains within itself the possibility of transferring to the communist position.
This position employed a wholly objective, fearless, ruthless and critical analysis of the social situation that was developing in the world of private property, especially in those countries where private initiative bad secured the utmost freedom from any external, legal kind of regulation, namely, in England and France. And so the criticism of communist ideas, so far as Marx considered it a serious-theoretical matter and not a demagogical-idealistic one, became a criticism of the actual conditions of life that gave birth to these ideas and aided their dissemination.
Therefore communism was even viewed by the young Marx as an ideological current arising out of private property itself. Thus the criticism of communism finally became a criticism of private property as the foundation of communist ideas.
This plan of critical analysis became central for Marx and served as the basic theme for the Philosophical-Economic Manuscripts. This work led him to the conclusion that those actual-empirical conflicts, in the soil of which sympathy arises for the ideas of communism, were not accidental phenomena, characteristic only of the England and France of that time but inevitable outcomes of private property seen as an international and general principle for the organization of all social life. Marx became convinced in the course of this analysis that the conflicts actually observable in France and England were, in essence, necessary consequences of private property; they were already present implicitly in the very principle of this private, individual kind of property.
If this is not understood, if it is seen the appeal of ideas have little basis in the conflicts and antagonisms in the world itself, then there is no understanding of the problem and why it will continue until self-destruction or resolution.