So, if a person does not believe in God, what strong, logical, reasons would you offer to talk him out of similar atrocities?
Terrorism due to twisted beliefs is a different topic; my question is about the source of morality outside a healthy religion.
If you say you "enjoy the spectacle of annihilating the human race", I think that makes you (your hypothetical character) some sort of a blood thirsty psychopath. A psychologist who have dealt with many psychopaths would probably be most qualified to deal with that type of mind. I don't think the rest of us - Atheists or believers alike would know how to effectively reason with a psychopath like that without prior training or experience.
However, this line of questioning seems to lead to something that says perhaps there's an answer in religion.
What do priests or clerics know about dealing with mental health? What's their verifiable, objective, independent track - record in the matter? None.
Therefore to get more to the point, Atheists do not have the answers to reason with an unreasonable degenerate but neither do believers.
The only hope however is science. If we understand human behaviour, human desires and psychology, then that is the key to solving this problem.
This hypothetical scenario is not a dilemma for the Atheist because it fits in well with the world view that no gods exist.
The fact that such insane people exist and have existed to me is a sign that a loving, caring god does NOT exist, unless he is actually such a psychopath himself who enjoys creating murdering psychopathic humans and watch them rampage and destroy other humans.