As for the rest, your logic makes no sense. You're extending to the killer a salvific exemption not extended to his victims. If we must by your logic assume that the victims have met their eternal fate because God employed the killer as the agent of His will, there's no reason not to claim the same role for the State in applying judgement; and the murderer has ample opportunity to make a full confession and perfect Act of Contrition, something of which his victims were almost certainly deprived.
Your viewpoint is reasonable, as well. I don’t think it’s necessary for me to say, “I’m sure,” in general or about particular cases, because I’m not in a position to make the decisions.