(This is a serious discussion. I'm not ranting, or accusing anyone. And regarding the examples I show bellow, they are by no mean represent the entire fandom of any show)

You must have heard of, or experience it yourself, the terrible fandoms that are My Little Pony, Undertale, etc. fandoms. These obsessive and uncivilized fans, they bombarding you with memes and rantings, slipping their favorite shows in every discussion instead of context, and might even harrass you for not sharing their opinions.

Is there something commons with these shows, howevers? Are these shows acting as beacons for all the sadistic, nihilistic, obsessive, and purposeless people out there?

In my opinion, yes. As you can see, the bad things committed by these fandoms all came from small groups of fans who considered themselves elitists, the only people who understand all the nuances of the shows. They've elevated their favorite shows into something almost holy, something that codified themselves as people.

Conversely, the show must be able to establish a distinctive culture surrounding it to foster these elites. It must be different, not original or popular, but just different from its contemporaries. The idiosuncracy could be in the theme, the story, the art, the voice acting, etc. Bonus points if these shows have theme that resonnate with the darker part of people, like racism, sexism, nihilism, atheism, etc.

Being debatable is another good way to have these elitists, though it is not always the case. This normally means that messages the show implies are debatable, such as the equality message of Steven Universe, or the nihilistic preaching of Rick and Morty. (Not that I am saying that equality is bad, but is that there are many people taking these things too far, and others are being annoyed by these people's altitudes.)

That is my opinion. What is yours?