Before I type this answer, here’s a disclaimer: I support eugenics. I believe it’s a fundamentally good idea, that there are too many people, and that there are way too many of the bad people. (And no, I don’t have any kids and I know that I shouldn’t have had any.)
The problem lies in implementation. The quotee here is talking about “slaughtering fools, morons, criminals and other blockheads”. That’s what’s giving him a hardon - the slaughter part. Notice also he’s talking about 7 billion people here. It also means that everyone remaining on earth would have to have indulged in an orgy of killing, making them the least desirable people to live by anybody’s definition.
Or how about the Infinity Gauntlet turns out to be real and Citizen Justin gets it? It’s a silly and stupid idea but still about as likely as any other scenario. As a way to thin out the population, Citizen Justin decides to vaporise anybody worse than himself, and snaps his fingers. Nobody dies because Citizen Justin has just become the worst person who ever lived.
I still maintain that there are people who are objectively worthless, and also that are kind and humane ways to reduce and improve the population over a matter of decades. Even if I were to explain these ideas, I would no doubt end up on FSTDT myself. Call me an immature edgelord if you will. But as the other commenters have said, the people who trumpet eugenics the loudest are always the first people who should go. It’s an unfortunate paradox of real life.