Humans hate captivity, no matter how comfortable. We need more, a purpose to be whole. He's been given an enemy to fight against that 90% of the world is ok with killing (Aryans, Whites, whatever you want to call us). So, he probably sat around getting fat and high and angry in his box he calls a home until he thought up some way to lash out.
Its like the NPC is stuck until the guard's death animation ends
He literally savours his crime.
Now he's going to heaven according to his religion.
Even according to Christianity, if only he believes in rabbi Jesus.