Does Atheism Lead to Nihilism?

[Mudhammam]

At each level, the motivating principle is self-interest, although at the higher levels it is an ‘enlightened’ self-interest.

The same can be said for any action or suggestion ever made, since you're broadening the meaning of "self-interest" by the modifier "enlightened," which is simply to imply self-interest that takes the interests of others into account as though they were one's self.

How is it even possible for to group particular things into universal categories within the framework of naturalism (your version of reality)? The foundation of your moral system crumples if it isn’t grounded in a metaphysics that is compatible with things, like human beings and moral agents, having essential natures.

I think people are content to imagine the worst possible suffering conceivable as essentially bad. From there it's not hard to work out a better framework that guarantees safeguards from the worst possible suffering conceivable and ideals that coincide with what a person defines as their real best interests.