Absolute morality?

I just finished a debate on morality with a moral realist. For those who don't know, moral realists are certain that there are absolute moral laws very much like there are absolute laws of physics. My own view is that morality is a human construct and mostly subject to the whims of individuals and society. That makes me a moral relativist. What scared me about the debate, and gave me reason to distrust realists, is the stance the bloke had on the idea of competing moralities.

My opponent made the comment that there is only one right way to look at any given question. Reasonable enough, yes? He went on to say that people who do not believe this one right way are delusional or self-deceptive. What? So now if you argue with the Right Way you aren't defending an alternate stance, you are mentally ill. He continued by saying that there is no requirement for external, objective proof of which way is the right way. So the Righteous can make up whatever moral law they want and they don't have to provide rational support for their law.

Taken together, these views account for all the evil that has ever been wrought on humanity. Stalin, for example, was convinced that those who did not believe in his moral crusade to help the proletariat were just misguided. He established "re-education" camps to help these delusional folks find their way back to the moral path.

This sort of absolutism is dangerous. This is what allows people to justify all manner of atrocities. Such absolutists are convinced they, and they alone, hold the key to human salvation.

If you want to claim your way is the One True Path you had better bring some evidence. Newton didn't just claim, "my theory of gravity is the right one!" He backed his claim with testable facts. Today, nobody questions the veracity of his claims. Because everybody can see he was right. If you want everybody to see you are right you better bring the hard facts so that people find it impossible to dispute your claims. Just saying, "because I said so" will not fly.