He turned into Sirius Black.

Yes, he did!

And now his mission in life is to go back to Earth to kill Peter Pettigrew.

He thinks that's an odd name for a human female but w/e it's better than 'that girl Thor hung out with'.

Yes, because clearly he needs to kill SOMEONE for landing him in prison

And possibly many someones on the way. I'm sure he's not too fussed.

Preferably ALL THE SOMEONES!!!

because killing people is a great way to work off aggression that would otherwise be used to do something bad.

Brilliant, flawless logic, that.

He could host excellent anger management classes. "Now, everyone take a knife..."