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.
No. You seem to presume that God can forgive your own sins, but not those of others.
The sin of Pride comes to mind.