Satan is what the christians call the evil god, or gods, a Satanic agent doesn't necessarily even know they are a Satanist, let alone actually worship Satan just they are manipulated to serve an evil purpose by Satan.
But based on that logic, any form of sin is an act of Satanic worship since every sin is enacted due to an evil purpose. Therefore calling someone Satanic is moot since every evil act is technically Satanic. What I call into question is what specifically about Karl Marx or his ideology is especially Satanic. As you have stated, his atheism isn't an issue either.
Torturing Christians is an evil purpose. Yes the communists also tortured orthodox jews, they tortured Buddhists too in the far east.
The Soviets never tortured Buddhists (I was just using it as an example that Russians didn't care for what religion you adhered to). They never came close to torturing Buddhists at all given that China was a vassal, not an integrated part of the main Russian Soviet Administration. Muslims, however, were most certainly killed. Muslims were the second most killed religious denomination in Soviet Russia with #1 being Christians due to having a higher population.
The evil god is not just against the Christians it is against human beings and also the good gods.
If you are, as you say, leaning towards Christianity or actually Christian (given the obvious religious zeal you have displayed upon this thread) then referring to any other religion's God, especially a Buddhist one, as "good" is very heretical. Now I am not quite the expert on Christianity but I think such a view would not be accepted by a majority of Christians in the world.
I don't think atheism is enough to explain the torture of Christians or Buddhists. We have lots of atheists in the UK but so far they are not noted for torturing Christians.
I already explained why they tortured Christians and Muslims, you're just too emotional to consider my explanation. The Soviets did what they did because they needed to consolidate their power. Religions, regardless of your opinions towards them, are primarily social institutions. Social institutions, by their nature, have large degrees of political influence and therefore, for a state such as the Soviet Union in which their ideology is it's primary social institution, religions and dissenting ideologies are seen as foundations for rebellion.
I'll have you note that the Russians tortured Christians, Muslims, Jews, Tsarists, Liberals, and all those who objected against the establishment in the same way, in the same prisons, and often, the same cells. The Soviets did not distinctly torture Christians for Christians were not special in anyway. To Soviet officials, they were just another bunch of rebels to mow down. As much as you would want to believe that there was some great evil which influenced the minds of the Soviets and commanded them to kill specifically Christians, such an illusion does not exist.
When I was a child I believed that there was a great force in the world which caused evil everywhere but that by doing good, you could balance out such a force and provide peace in the world. When I grew up, such notions of justice became more complicated. I began to realize that there is no force which permits evil and there is no opposing force which seeks to vanquish it. There are only people. People who are terribly, terribly alone.