I once babysat these two sisters, the older was 6 and the younger only 3. The younger was a calm kid. She would torment her big sister by pulling her hair when she slept, pouring things on her etc., but never in an angry or excited manner. Just to kind of see what happens.

I once gave them both a toy and the younger one informed me she wanted both toys, and she wanted her sister to have none. I told her that wouldn't work. She said "OK. I'm gonna wait until you go to the toilet and then I'm gonna take the toy from sis and break it."

I shushed her and thought no more of it - she was three! - and we went on with our day. Many hours later I had forgotten all about it and I went and used the toilet. When I came back, she had taken her sister's toy and destroyed it. She looked at me and smiled and said "I told you I would do that".

That kid made me deeply uncomfortable. Later on she'd also torment the family dog and hide feces in her parents' room when she got mad at them. She's in her 20's now, I wonder what became of her.