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.