Probably this is already a really basic psychological concept that I re-invented, but anyway.

Sounds like cognitive dissonance to me. I get, I also used to do the ”actually vegan diets aren't as good as you vegoons think” stuff before I figured out that the best way to get rid of cognitive dissonance was to just be vegan.

I never would've gone to spaces specifically for vegans to proclaim how awesome meat was, though. Do these people think there's a Medal of Edgy Contrarianism in it for them or something?

It's called moral envy

Right?! I used to think a lot about that kind stuff a lot and I have theory about it: They see a vegan simply existing and (maybe subconsciously) think "shit, not eating meat etc kind of makes sense" but then instead of learning from it, they take it as a personal insult almost. Like "fuck you, you're not a better person than me". Then the initial reaction is to deny and deflect and throw shit, so you can delegitimise the argument and won't have to think about it critically. Same reason that people who like to drive cars a lot might dislike cyclists, or why some people are instinctively annoyed with people who are athletic. And now that I think about it, the same thing happens when libs face criticism from the left. Probably this is already a really basic psychological concept that I re-invented, but anyway.

They can't be doing the wrong thing. They're the main character.