When it all comes down to it I think the ones who are the scariest are some of the ones who hurt the most. What a remarkable defence for a little child... in a situation where you have no power at all, no other means of self protection, and the prospect of years and years of pain and abuse ahead of you, you become the most frightening, indestructible harbinger of pain and destruction to others.
There is no better way for a powerless child to feel at least some sense of control and self agency.