nobody can say for sure what is behind it... what would cause someone to have such anger.
but the possibility that that bully was bullied by someone else first may be a real possibility... i would look at his parents for starters.
If parents had bullied him, it was there way of conditioning him to believe he was not accepted for who he was... he then became like them...
Bullying others is a way to get revenge for the way he was treated...
Therapy needed most assuredly.