JonathonSpectre posted:

OK so if you have a wealthy megalomaniac who's playing the REALLY long game...

... back 30-40 years ago he quietly gained a controlling interest in several large multi-national beef corporations, and then deliberately saw as much of the herd as possible infected with mad cow, possibly a genetically-modified mad cow that doesn't show up on the current test or onsets much slower in bovines or something so it isn't detected. You could sci-fi this by having it be a made-up disease that uses some other food vector but honestly mad cow is one of the scariest loving things on Earth.

Decades later, the disease begins to manifest hardcore as everyone who ate any of the infected beef sees their brain begins to slowly disintegrate and society collapses in a few months due to massive numbers of critical workers barking at the moon and talking to their dead ancestors. You can even make the villain an apocalyptic religious vegan or something.

"THE COWS SHALL NO LONGER FACE YOUR HOLOCAUST. YOU SHALL NOT CRUCIFY MANKIND ON A CROSS OF STEAK."

Now you have your large population percentage dead (and it will hit harder in the more affluent meatasaurus nations, causing things to go to poo poo faster) and your villain can gloat about how he gave the people of the world a choice.

There's also no way anyone can do a loving thing about it once it's done. There's no vaccine, no cure, no hand-wavey Star Trek bullshit to save the day. The damage was and has been done, so unless you can invent a time machine the human race is going down hard. You could even have him say, "I'm not a comic book villain, Rorschach. I did it thirty years ago."