Because it isn't just.

[Oh, here we go, even if he's trying to pick his words so Tim doesn't burst into flames again.]

I'm a bounty hunter, but I'm more importantly a judge of the punishment for the crime. My primary objective is just to make sure the balance is restored- the dead stay dead, the living do not meddle with that. It brings me no joy to murder desperate people who are backed into a corner.

[And he doesn't. He didn't kill Lucas despite coming within an inch of destroying the world. He just needed Maureen to come back with him.]

Suddenly having a power thrust upon you that you cannot control that strikes you down? It's simply not fair. No one should die like that.