What makes a person willing to kill to exact revenge or to protect something? Michael had such a nature. To that extent he was just like Vito when Vito was younger. Sonny was like that only emotionally. He didn't kill as a strategy. Michael and Vito had substantive reasons to kill. Remember, in the novel, Michael tells Vito that it's not just out of revenge for Appolonia and Sonny that he is doing this. He told him that it's also the right thing to do. So, his nature is to view murder as right.