I don't believe in evil. There's only what we do. So, I don't believe that Mike turned to evil. He did things that we characterize as evil. He didn't think they were. He thought they were justifiable. Concocting the murder of Sollozzo seemed justifiable to him just as his divorcing kay and almost turning her children against her seemed justafiable. If there is evil, he always had it in him. It just took circumstances to prompt its expression.
According to Phil Zimbardo evil is related to power, the will to dominate people.
http://www.youtube.com/watch?v=OsFEV35tWsg(This is just a little summary of his 500 pages book, which in fact is a 1,000 pages book because of the small fonts.)
In my opinion there's a significant change in Michael towards killing. When Tessio and Clemenza discuss the killing of Philipp and Bruno Tattaglia, Sollozzo and others, he asks: "You kill all those guys?" Two or three days later he's ready to kill Sollozzo and McCluskey, and he knows that this will change his life forever. He knows he has to disappear, he has to leave his love. He knows he's acting against the law.
Zimbardo says, we all have both sides in us. The question is, are we ready to resist? We don't know what Michael experiences during WW2 and what he really did there. Very probably, he saw injustice, he saw cruelty, he saw people dying. Definetely more than shooting people a mile away.
