You don't have to identify as "belonging to a fandom", being part of a compact group where the actions of a few define the others. I like chocolate. You probably like chocolate. You know who liked chocolate? Hitler. Bam, we're both terrible people. Same logic apply here

Fanatism, whatever it's shape will ruin stuff, even if it was great originally. Undertale is a great game, being swarmed by angsty teenagers and cringy shitposters can't alter that. You simply have to ignore them and put them in your mental "Trash" category.

If you think a community has become toxic, then it's the community's problem, not the games'. Don't go to their forum if that's the case, or accept that a very popular and/or passionate topic will attract all kind of people, including cancer-posters. Same thing for Minecraft, TF2, KH, etc