I like when the villain becomes the hero out of exhaustion. The good guy and bad guy just get tired of not getting along, but they spent so much time together that they just get along.
This happens in Good Omens although it happens in the intro as context for the rest of the story.
I suppose you could argue this happened to Dr. Eggman since apart from Sonic Prime he's been a complete joke in every piece of media he's appeared in since like 2008 and in almost every piece of media he's appeared in since 2013 (including Sonic Prime) he's ended up teaming up with his "archnemesis" Sonic to defeat the ancient evil of the week he awakened in the hopes of finally defeating him
Venom is still the Villan in most cannons, but I think the reason he's the hero in his movies is 1. It's his movies, of course he has to be the hero, and 2. He hasn't meet deadpool in his movies.
Iirc Deadpool was actually Venom's first human host.
Deadpool having 3 personalities was too much for Venom which is why he was so fucked up when he latched onto Spiderman. He was literally trying to put his head back together and that's why Peter was able to reject him.
Galactic population also would have been right back where it was in, at most, a few centuries. It was a stopgap solution at best. And his comics motivation wasn't anything noble; he just wanted to bang Death and thought omnicide would turn her on.
Thanos' motivations should be seen more as a fanatic rather than the hard but necessary choice. Tons of ways to achieve his goals without killing half the universe.