Peter's belief in his moral superiority as a leader and a Lord is contradicted by events. Matthew has earned loyalty. We see him earn the loyalty of Anthony (and it would be good to see Anthony risk his life for Matthew after that humiliation) as well as the loyalty of the common boy he interrogated, then took in.
Peter, on the other hand, cannot earn loyalty and fails when he tries to buy it. He destroys his tools. Even if Peter does not consciously recognize the contradiction between his jealous assessment of Matthew's character and the Matthew he is fighting, his inability to fight on level ground with Matthew must rankle at every turn--and he must be evading and hiding from the measures of character.
I think if you keep this in mind, you won't need to add any additional seasoning. Jealously becomes the deadly sin of Envy, and that progresses to raging, irrational anger. Just let us see that, in microcosm, and let us see how Peter uses the Anger to defend against the truth.
If you can't have Anthony strike the killing blow, can you show Isaac (is that the name) risking his life in loyalty, and, injured, still showing more concern for Matthew?
Death by sepsis for Peter would open the door for more character moments between your other characters. It would let him rage a bit in nominally lucid moments. And, in the end, he might slink away to die, to 'escape', leaving Matthew to drive away the animals feeding on him, in order to give him a decent burial. (Driving an animal from its meal can sometimes only be done by killing it.)
