The Tinkerbell Effect.

[Esquilax]

So history's monuments to moral relativism (Pope Urban, Stalin) serve to teach us that nothing is wrong if you believe it isn't? Or were they deluded and truly in error?

The point is that it's more complex than a binary right/wrong system. It's a matter of evaluating the entirety of the situation; murder is wrong because it harms people and allowing it to happen has a direct harmful influence on human society, and can thus be seen as objectively wrong. However, there are times where we allow it to some degree, like self defense, sometimes even capital punishment (though that's a complex issue.)

It's a complicated continuum of costs, benefits, and a number of other issues, and moral relativism is incorrect merely because it places some acts that are objectively harmful to a degree that they should be stopped into a category where they're sometimes acceptable. You can be as relativist as you like, but you're still bound to a physical universe that has truths one can state about it, and therefore has actions that are objectionable.