Your premise is flawed, which is why you’ll never get an answer to your question. It’s like the tired old atheist argument of *If God can do anything, can He make anything to big for Him to lift?* It begins with a false premise and so the whole point is moot.
No such thing as a *Christian mass murderer*.
A person can be a Christian, in which case they are not going to go out and mass murder, or murder anyone for that matter.
Or they can be a mass murderer who repented and has become a Christian, in which case, he is a new creature in Christ and God does not count the sin of mass murder against him any longer.