Nice people tend to gain lots of respect from others, so when they get mad, people tend to fear them a bit because they have so much respect for them.
Another reason why nice people seem "scarier" when they go nuts is you're so used to that nice version of them that you're unprepared to react to the angry version. Since you hardly expect them to go mad, you don't know how to deal with when they do. It's out of your power, so it makes you frantic. However, this only applies if you're a jerk who likes to pick on nice people because jerks usually have this wrong perception that nice people are easy to walk over.
