There's a school on counseling called Reality Therapy that I don't really like as a whole, but it does make an important point: If you expect that other people will always act in ways that consider your emotional needs, you're setting yourself up for a world of hurt. Even more so than personal relationships (where I think it's reasonable to expect someone to act with concern and consideration) this applies to media and art. You can't expect that your headcanons will be respected by creators of the source text.
I am 110% behind people projecting onto fictional characters whether it's seeing aspects of themselves or aspects of other people in their lives, including projecting onto villains. I do this, again including villains. I don't think it's unhealthy, and I believe it can be highly therapeutic - especially if combined with creative activities.
But, I also want people to be able to bracket that projection. If you're projecting on a character that is not your own and is part of an unfinalized work, you are exposing yourself to the creators doing something with that character that you don't like. And if you've bracketed your projections as thoroughly valid but still separate from the canon character you've got a buffer when that happens. Sure, you may be pissed, but if you can distance yourself, you can hopefully avoid the highly unpleasant experience of feeling personally attacked. Because the creators aren't responsible for the emotions you've projected on a character. Their creative decisions are not about you, so there's no reason to feel personally attacked. And, ultimately, it's highly unlikely that taking a creative decision personally will be psychologically helpful.
The good news is that just as you can't control the creators’ decisions about a character, you have the power to engage in creative, transformative actions with your version of the character to work through emotional needs you projected. Develop your headcanons, make art, write the redemption arc you want, create elaborate AUs, critique the source text. Do what you need to do to process the emotions you were projecting. And if someone tells you that your works aren't valid or are “problematic,” tell them to fuck off. Your rights may end at the tip of another person's nose, but the reverse is also true.
What I will suggest is that if you want to create a climate where people are considerate of each other, practice that consideration. You only have control over your behavior. Be courteous within fandom whether you're happy or not with the creators and we're more likely to collectively experience a climate of courtesy.
Oh yeah, that last bit goes for all of us, me included, because snark is one of my main character flaws.
