Sports. That shit goes to insane levels.
See, every group has toxic fandoms, some more than others, but movies, gaming, music are pretty leveled between each other. Anime? Might seem intense on the internet, but those weebs dont even know what other human beings look like. But sports? That shit gets to insane levels.
In some countries, like my own, the fanatism gets to a point were people have been killed in stadium fights and such. You dont hear that shit about almost any other hobby. Nobody ever said "16 people died in last night Smash Bros tournament after a fight broke because fans of Luigi were tauntin Samus fans". That shit is sports-exclusive.

There's only one fanbase that comes to mind for me, and it's more problematic than every comic, game, movie, and consumer product fan base put together. I'll leave it to you to guess which fanbase it is, other than to say they really like baseball caps, and I don't mean the Red Sox.

Steam lol..
Joking, politicians fanboys are worse than them and soccer fanboys are even worse since some of them even commit murder and severe violence with to much frequency.

But whenever there is a mass shooting media try to blame gaming. There will be a time when the hooligans and the like will blame Fifa and Winning 11 for the violence.

Sonic Fans.

Listen...I like Sonic the Hedgehog. Used to be my favorite platformer series. But Sonic fans today are just strange. Sonic could break his leg and Sonic fans would be like "That's so f*cking RAW, Sonic broke his leg!" and I just can't stand it. They're so annoying. They get hyped over literally anything.

Jesus

This isn't really a fanbase, but I dislike when people would call something the killer of something. Like a WoW killer for example. I also dislike the culture to call something dead, I know most are just haters but when even former fans say that it's just petty. If you stop liking it, just move on, I don't understand people that take their time off to just stick around to complain, most likely to feel assured they made the "right decision" and think they're better than others. Criticism is different. I think this can be said to all fanbase as a lot of people are obsessed, like kpop fans are especially worse when it comes to who's best, charts, views, etc. The feeling that we can only like the best or most popular is competitive at best, unhealthy at worst.

Furries. They should be put on a list and monitored.

I don't really hate fans, they are passionate about their thing and without that passion, these things would not be the things that they are. As long as it's a true fandom, and not a political one.