Gonna ruin this post, sorry, but it reminded me of this (article is in spanish).
Basically it’s about this 17-year-old girl (she was adopted when she was a baby, and her parents gave her everything she wanted), who murdered her parents because they wouldn’t give her a car. The psychologists are really surprised because she got 9 out of 9 in a psychopathology test, which is the highest you can get and means that she’s a coldblooded murderer, and feels no remorse at all about what she did. She planned it for about a month, and because she’s really small (4’11”) she convinced her boyfriend and another guy to help her.
They killed her parents in her house and put them in a car to incinerate the bodies. In the end, it was the boyfriend who couldn’t take it anymore and confessed. She had been giving a very calm testimony up until then, but when her boyfriend told the police, she admitted it, and said that she had felt “free” when she had killed them.