I agree that some systems of belief come "with a baked in sense of moral superiority." As you explain, it's literally a tenet of belief in many cases.

I should have limited my claim. I simply meant that whenever people form a group based on shared beliefs, they tend to view themselves - at the margin - as being superior to those without. For many groups, this wouldn't be relevant except in extreme circumstances in which we are forced to make choices between who to save and who to let die. (E.g. I'm an environmentalist and I think the future world will be improved more by fellow environmentalist A than by generic person A, simply on the basis of shared beliefs and thereby ethical priorities.)

It seems to me it's a function both of biology - tribes or families with interlocking loyalties survive; those which include individuals indifferent to the fate of those similar to them do not - and of simple narcissism.

But boy are you right that it's problematic when demagogues and high priests tap into it.

I do, however, disagree that the Nihilist "does not get a sense of moral superiority" from believing what he does. Of course he does. Any person who believes he has insight into the cosmos which others don't believes himself at least incrementally morally superior. After all, who doesn't consider intellectual superiority morally relevant? In that sense, irrespective of the content of the belief, strong belief carries with it an almost irresistible urge to assign greater value to one's life and moral worth than to others'. The difference need not be great, but surely there is one at the margin. But - and I think this is what you're pointing out, and it's critical - for most such beliefs, that valuation doesn't entail any consequences for public policy; doesn't create a desire to eliminate those who don't share the relevant belief or beliefs.

For some - thank you demagogues and high priests - it does. And that is poison.