"There are no terrible fandoms. Only bad people who happen to be a part of a fandom"
This makes sense to me. Specifically for most fandoms with core subjects that aren't necessarily malicious themselves.
No matter what a fandom is about, be it a TV show, artist, or philosophy; the fandom is pretty much what the fans make it
That means that terrible people can enter a fandom and give it a equally wretched atmosphere as themselves. However, this is counterbalanced by decent people in the same fandom. And every single fandom has a balance of regular people and shitheads. It's just how things are
That means that it would be unfair to judge a whole fandom as terrible based on a terrible induvidual you found within it and their usage of the fandom. It's just that the terrible people in fandoms tend to get more attention and focus, simply because they are terrible.
It is also unfair to judge a fandom as terrible simply because you don't like the subject yourself. That sort of thing is subjective. It's better to look at individuals in the fandom on a case-by-case basis
On the flipside:
It is possible for a fandom to be completely populated by terrible people and have no counterbalance of decent people. That can be achieved by a fandom having a truly terrible core focus that only attracts terrible people. In such a case, it is possible for that fandom itself to be terrible.
Example: James Holmes fandom. If there's a fandom that is essentially terrible just by itself; That's one. But even then, that was a terrible fandom made by terrible people to be terrible.
Possible moral to take from this: Terrible people can make a fandom. But fandoms don't make terrible people.