No one is born bad, Every one has their own good reasons to play the role of evil in someone's life. I guess this is the reason the same people who is in good book of one may not be good in others .But you turn to an evil when you start being so selfish ,when you dont feel empathy for the same ,you go on doing bad to that one person .so you understand how does it happen? Good to bad and bad to being evil .
*but no one is born bad*

- Simran