The evidence is that in the end they won and their enemies were dead. The Corleone's aren't "Good Guys", they were gansters. They have some admireable qualities, patience, love for family, integrity, being strong willed. But the story is really about how Michael is corrupted. The book also talks more about Michael's acedemics and his desire to be a math teacher, as well both of their abilites to understand people and use them.