Evidence for civil war is lacking, and Jews didn't come to Egypt with nothing. The tribes were very rich with livestock which they took with them at the invitation of Joseph. Also many that left with Moses did not believe in one god as was proven by the Golden Calf.
Evidence of civil war is present in Egypt and the Egyptians documented all their civil disturbances. At one point, the south rebelled to the point were all the empire was about to collapse because of it.
When the Hebrews came to Egypt during Joseph times, they were exhausted. Had very little food and money. That is the reason why they came to Egypt wasn't it? So the idea that they came to Egypt as rich people is not true. If you want to test this theory, you can go and live with the Bedouins for a month or so and see how they stock their food. Its almost impossible for people to live in tents and be able to store large amounts of food. At that time, they Hebrews were nomads, Bedouins, moved from one place to another to feed their stock.
As for the people that left with Moses, they were ALL Hebrews. But not all Hebrews believe in ONE GOD do they? Not all Israeli's are Jews now and there are many who are atheists. Same back then.
The bull was made not taken with them from Egypt. Moses would've never accepted an Egyptian or a pagan God taken with him on the exodus. And because of this act, making the bull, he ordered the slaughter of many Hebrews. When they refused his command, he ordered the Levites to carry it out, ordering them to kill even family members who joined in this act.
That is not brutality, but purification. These people left Egypt and witnessed one miracle after the other and yet still chose to worship a bull, they don't deserve GOD's mercy.