All good points, olivant. Out of all the sons, Michael is the one who most resembled Vito IMO, which is why he is the main protagnist of the story.
Even though it is not shown in the movie, I always thought that Sonny's aggression stemmed from watching Vito kill Fanucci. The book mentioned how Vito came to realize that Fanucci wasn't as powerful and well-connected as he seemed. But Sonny was not privy to his father's thoughts about Fanucci. He only witnessed Fanucci's murder, which I think led Sonny to believe that violence was the way to get things done.
Of course, the novel goes into things so much deeper. It says that Sonny had a quick, hot temper. I think that is illustrated in the film when he visits his sister the first time and sees her black eye. Then there is a deleted scene in which he starts to leave his father's presence to go after Carlo and Vito tells him to stop. So, I conclude that while witnessing Fanucci's murder may have sanctioned Sonny's future violence, its motivation came from deep inside him.
