Wow, there's a lot of finger pointing at different generational demographics here over something that's structural to Reddit.
Stupidly big forums + up/down votes dictating what actually gets seen is a recipe for dunking, sarcasm, and generally shitty behaviour.
Onces there's more people in a community than people can actually remember the name/pfp of, then other members stop being people and start being either an audience or cannon fodder. Couple that with the fact that people love a good snarky comment or rhetorical thrashing, and that leaves busy spaces as prime real estate for smack talk showdown.
On top of that, there's simply the fact that anyone not trying too hard to get noticed just doesn't get heard at all. Taking the time to post something thoughtful when literally no one is going to see it is a fool's errand, and not worth anybody's time. So, you either waste your time and become increasingly embittered, or you don't, and just say vapid but snappy bullshit.
Then there's the fact that moderators are overwhelmed by groups that large, and will default to mental self-defense by doing things like banning without warning, not being transparent, not attempting deescalation, etc. This creates a gulf between the community and the community managers, which furthers the dehumanizing dynamics (and leads to people seeing moderators as power tripping narcissists, rather than tired and fed up people).
We simply didn't evolve to empathize with, listen to, or manage 600,000 people at once. We did, however, evolve to try to win popularity contests and define in-groups and out-groups.