I had a cousin who was adopted and was a complete piece of shyt, and every Christmas I'd end up seeing him. It completely ruined multiple Christmas holidays until he stopped coming around us when he was 15 or so. Whenever I was left alone with him he would try to fight me or fukk up anything I was doing. I beat him in SF2 once on Sega Genesis, at my grandma's house, and he broke my controller and hid his so I wasn't able to play anymore. One year he kicked my other cousin, who was about 6 years old, down a flight of stairs. She almost broke her neck. My grandma could tell by my reaction that he did it, but since she was too scared to say what happened it was swept under the rug.
He ended up constantly in and out of prison and is now serving a long ass sentence.