For Vito too, the move into the criminal world was slow. It started when he took Clemenza's guns. (Evil always starts with small steps.) He was present when Clemenza stole the carpet. Later, he joined the robberies of Clemenza and Tessio. There may be some reasons to justify that but you can't call these cations "good". The first murder Vito commits is the killing of Fanucci. And in the given situation of anarchy in Little Italy at that time, where the government had no intentions and no means to bring peace and justice to the neighborhoods, even that murder may be justified. But Vito was already at a point where he wasn't free of selfish reasons. Later, the killing of Fanucci was viewed as an act of heroism, but in fact: one robber killed the other. In the film (not in the novel) he seemed to acquire a taste for killing, so he went back to Sicily to kill those who were responsible for the killing of his parents and his brother.


No difference to his father. Vito did not want to be a Mafia boss. Mike thought that by killing Mac&Sol he would do good. As you say it was like "killing Nazis". But that was the first step across the Rubicon. Probably, he subconciously chose to be part of the game after the shooting of Vito when he joins the discussions of Sonny, Tom and Tessio.
The social pressures - family, situation, role models - were so strong that it looks as if he had no choice. But he had. At any given time.


Good thought.