On One Piece how do you feel about major Villains turning to major allies?

Luffy never killed his opponent, and there's a reason why the author do this. The author stated in the interview that crushing an opponent's dream is more interesting that killing off a character like any other Shonen does. Crocodile's dream to rule a country was destroyed by Luffy and he probably have a new goal when he decided to helping Luffy. I think he probably have a hidden agenda like escaping from the prison with Luffy later on.