It doesn't really work that way. Before a person is saved (as we put it), a person has to repent in his/her part. If a serial killer were to repent totally (has that ever happened?) then the person would no longer be a serial killer.
It's like a criminal getting out of prison and actually turning over a new leaf, rather than going back to his/her old ways. That person is called an "ex-con" but he or she isn't a convict anymore. He paid the price for his crime. If he or she breaks the law again, then he or she will be put back in prison.
As for our belief serial killer getting off Scot-free while someone innocent gets punished, I don't think it works like that, either. Jesus said that people will be judged "according to what he or she has done", not that they are automatically punished for anything. Just a little thing people often don't pay attention to.
(Sorry, I can't recall which verse it was that said that, I will look it up and post it later, if you like).