This is the same question that always pops up in my mind.I thank you for bringing this into discussion.I think it is because we are not pure black or whites.What I mean that w are not pure good ones.We have an evil side too.But it is us who decides whether we should let that side sleep for the entire life time or are we going to take it out.But being humans we take that side out from time to time.But our fear to do sin keeps us away from making a mistake.But not all have that fear with them
-cheers-