Wow, there are too many cases of people scared of toxic fandoms in this forum...
There are good people and assholes everywhere, and sometimes, jerks like your work.
It`s a curious and concerning psychological phenomenon and i think it would be interesting understand the reasons behind.
It would be great if an expert starts doing a research to understand how and why it happens.
Obviously,toxicity is because some deep seated insecurities from these people.
I think it is because they project themselves too much into the work, a sense of entitlement and dogmatic attachment.
This tweet exemplifies the sense of entitlement.
The projecting themselves too much, can be seen in obsession over certain characters or particular ships. Also, this may cause the attempt to control where the story goes.
The dogmatic attachment can be seen in people obsessed over the message of a show. They basicaly treat the show as a sect where they belong. And to find "status" in this community, they try to show how devote they are to the message, imposing it over people or feeling atacked by small and petty reasons.
A fandom is great if they share their love for a comic/tv show/movie/book/etc... in a healthy way. But not everyone has a healthy life and try to find some sort of escapism somewhere.
They may be hurt or insane, but that doesn`t give them the right to hurt people.
Remember: be confident about who you are and your work. The customer is not always right when it comes to storytelling, and do what your love. This is your story and you are in charge. If they want to do their version they can do some fanfiction.
I don`t have all the answers, handling troublesome people is a discipline in itself. But like anything, it can be learned and improved.
Bonus:
This site has a list of many kinds of annoying fans and examples in case someone wants to know better about the topic of.