Hear me out in this early bird thought of mine
You're an eugenicist, you only want the physically best amongst humans to survive (exceptions made for the fat, rotten, adrenochrome-guzzling billionaires that bankroll you and their more-inbred-than-the-House-of-Hapsburg-living-in-Alabama families of course)
So you make up a drug to kill and sterilize as much people as possible, and the mockingbird media and the jackboots will do their part to force as many as possible to take it.
Aaaaaand it's killing young, athletic and perfectly healthy individuals while the 400-lb neon-haired SJW's with a permanent double IV drip of Ben'n'Jerries and the malnourished vegan soyboys are still out there.
Makes even less of a sense if you're a white supremacist, in which case the shots should be completely decimating places like South America, South-East Asia, China and India.
What gives ?
The major flaw in your logic is in your second sentence. They have no interest in the best of humanity. Their only interest is to destroy you, so their incest filled class stays at the top. Even if it means destroying the gene pool worldwide. You've got to remember, these people are psychopaths.