ok first of all: NINE HOT
second, iirc the whole incompatible energies thing is deadly, so the brother is probably gonna die whether nine actually decides to kill him or not
on one hand in kinda hoping he does die, mostly bc i really dont see a way for them to keep him alive and ensure he wont come back for revenge (unless they go the "oh we won't kill him but this whole thing destroyed his werewolf powers forever so it's cool" route, which would kinda feel like a cop out imo but we'll see how it goes)
on the other hand, no matter how angry nine is now and how much he hurt him in the past, im not sure how nine will react to it later, so i just dont want nine to kill him with his own hands bc i love him and i dont want his asshole twin to be angst material later :,V