Verified answer

Born in an orphanage, always weak and picked on...

I'd elaborate on childhood trauma, maybe his dog was hit by a car/he was beat up by the bully constantly/he was abused by someone.

Perhaps later on he tortured small animals or became the bully in high school. Does he have any siblings? Was he mean to them? Did he take his sister's dolls and cut their hair? Melt his brother's little green army men?

If this is a made up story, he could have been abused or abandoned as a child and made fun of and the black sheep as a teenager. You know the usual wedgie and swirly bully stuff. Then make him grow bigger and taller that everyone else in HS and he can be the bully then, only take it further than small pranks. Make him out to be a mental patient kind of person by HS.

Good Luck!!

Maybe a parent or close family friend did things to him as a child. Of course the offender was well trusted, so the family never suspected it. The continued to trust this individual and leave this child with him for long amounts of time. Eventually, the pain bottld up so much that one day he snapped and began a killing spree.

he was abandoned when he was a child and he had to live on the streets and kids always insulted him....