Good guys who turn bad and eventually cause an apocalypse and/or genocide.
1) Paul Atreides: escapes from the Harkonnen invasion of Arrakis, becomes leader of the Fremen and sparks a jihad across the universe.
2) Glenn Talbot: US general who helped take down Hydra-invested SHIELD, allies with the rebuilt SHIELD until being shot, brainwashed, and infected with gravitonium, influencing him to destroy the Earth.
3) Seymour Krelborn: flower shopkeeper assistant who finds a sentient plant, Audrey II, gets slowly convinced into killing people to feed Audrey until they’re big enough to take over the world.