@Octogon: Man this BB is quite a complex character. I wish more villians/antagonists were like this. I just can't seem to put a finger on BB because he has so many sides to him. On one hand he is Luffy, on the other, he'll destroy you if you bother him.
My only guess is that BB, is a person who cares for himself and his nakama alone. Not evil, not entirely good, just looking out for himself and his friends.
I would however like to know what the circumstance were for his departure from WB's crew besides killing a crewmate. BB is not the type to kill his own crewmate (atleast for no reason). There must have something to provoke him or force him to do something like that because BB probably could've just left WB's crew without harming anyone. Interesting mystery indeed.
I predict that should Ace be defeated by BB, Luffy will find out at the begining of the next arc or afterwards. Since those are usually the times when we get the biggest revelations.