They should definitely be warned, and perhaps be put into therapy with a professional, but self harm isn't social darwinism. It's just a mental health issue.

I agree that self-harm isn't social Darwinism. People sharing amusing stories of self-harm and chuckling about "cleaning the gene pool" absolutely is.

Natural selection is not the right tool to downplay and excuse harm to others, and has nothing to do with the scientific theory of evolution.

I don't think the GP was referring to social Darwinism, just plain old Darwinism, of the "Darwin Award" variety.

It's not related to the science of evolution, it's using the principles of natural selection for social commentary.

Yes, it is, at least to the extent that intelligence and common sense are selected for, and their opposites are selected against, in our current selective environment.