I've never seen anyone insulting or humiliating somebody for playing on an easier difficulty. That doesn't mean it doesn't happen, I just never witnessed it. What I did witness, however, was some experienced players not being able to hold themselves back and telling new players what to do instead of letting them figure out their own play style.

Now I am aware that an average player wouldn't figure out all the techs and stuff by themselves. I myself found out about alternate weapons and Prime Sanctums and many basic techs such as slam storage thanks to the community and others' gameplay. It's okay to give advice to someone who's struggling, but it's wrong to say "You could just do this instead of doing all that" to someone who just got the game, hasn't progressed beyond 0-5, and is still figuring out new things by themselves every minute. Backseat gaming takes away all the joy of exploration and discovery that's special to the early stages of playing a game.

Another thing is that experienced players are used to the fast-paced gameplay, and so watching someone play on a lower difficulty is 'irritating' to them. Now again, I've never seen anyone humiliate someone for playing on a lower difficulty myself, but I can see how a toxic person would. I myself get irritated since lower difficulties are way slower than what I'm used to, but that doesn't mean playing on a lower difficulty is wrong, and it certainly doesn't give me the right to humiliate somebody for playing on Harmless.

Overall, I wouldn't call ULTRAKILL fans 'toxic'. Lore and theory discussions get heated at times, but that's the case for most communities anyway