And who turned him into an animal? Unless he's got a purely physical/chemical reason for his actions then someone, probably a parent, caused him to be the way he is. Do you not believe that if he is taught what he should have been taught from birth that he can change? If he hasn't been taught right how is he to know right? Is he to be put to death because of the sins of his parents?
"Is he to be put to death because of the sins of his parents? "
The parents didn't violently beat their infant child to death with fists, feet and a 2x4.
The parents may be partially responsible for influencing the overall behavior of their child but they are by no means responsible for that individual’s act of murdering his baby sister.
Responsibility always lies with the one who commits the act. In this case it is a 7 year old child...not the parents.