You can make arguments for it starting with "killing billions of people would definitely be bad" or "some fraction of those people will likely share my genes, w

You can make arguments for it starting with "killing billions of people would definitely be bad" or "some fraction of those people will likely share my genes, which I have a biological drive to pass on". It encodes a slight bias towards human existence being a positive thing for us humans, but I don't think it's the shakiest part of that reasoning.