IMHO, even a mass murderer can be forgiven by God if he recognizes his sin, truly repents, and prays for His forgiveness. But that doesn't mean that he can or should be forgiven by man.
David Berkowitz, a.k.a. the "Son of Sam" comes to mind. It is possible that he has truly repented. If so, then I believe that God has forgiven him. But that doesn't mean that he should ever get out of prison. A truly repentant murderer can do a lot of good spending the rest of his life in prison.
I agree.
Even if Pol Pot converted he still deserved to be deposed and executed.