One of my pet peeves when reading fan fiction is when a character, usually someone who is the other possible love interest, is villanized to make another character look good or to make a ship seem more plausible. Not only does it often come with out of character writing for said character, it makes me feel like the person who is writing is not all that sure in the ship they are writing.
You do not have to villainize a character to make your main character, or main ship, look better. If the character is a villain already, then at least there is canon backing to your character being a villain.
My most recent example of this was in the Arrow fandom. I am a big fan of Felicity and Laurel, and of Olicity. However I found way too many fics that villainize Laurel. Laurel has her own character set-backs, but she is no way a good choice for a villian in an Olicity fic. And I’ve seen it likewise when people decide that Laurel/Oliver should happen, and make Felicity into a villain or an unhealthily clinging person. None of it makes sense with the characters we see on-screen, who are in fact friends and strong women.
Another example is found in Doctor Who. Martha and Rose are alternative villainized depending on the ship choice of the writer. Honestly if you don’t like a ship, don’t write it. I’ve found most of the time the ships resolve themselves around each other and you can avoid the other alternative fairly easy without villainizing a character. If you must deal with a ship that you dislike, resolve it between the characters and move on. Most of those reading are most likely already fans of your ship, and you don’t have to prove to them why your choice is better than the alternative. That is best left to fandom meta essays.
I’ve also seen this with just characters. Most often female characters, but occasionally male characters. They will exaggerate bad qualities to the point of OOCness or completely villainize a character to make their favorite look better. Your favorite character can be awesome all on their own. That is why they are your favorite. You don’t have to miswrite a character to make your fave look better. Again, most of your readership most likely already thinks your main character is awesome.
For a good example of how to deal with a love triangle, there is The Hunger Games. The love triangle in that series is between Katniss, Peeta and Gale. Peeta and Gale don’t fight each other over her, and both characters are allowed to have flaws and good things about them without comparing to each other. In the end there is a little comparing when she finally makes a decision, but for the most part all three characters are allowed to exist on the same side. Neither is completely villainized either.
My suggestion is if you really don’t like a character, have them take a trip, or be otherwise out of the picture of the story you are writing. If you feel you can write the character well, and not villainize for the sake of making them look bad, go ahead and write them. Take a look at canon. Be honest with yourself as to why you are writing the character the way you are.
If you are just writing angry fic about a character you hate, label it as such so fans of the character know to avoid it. And people who may hate the character but prefer to stick to canon can know that going in. For example, bringing back the Arrow fandom, I have seen people post “Anti-Felicity” or “Anti-Laurel” as a tag, letting me know that their story is not favorable to that character. So I can avoid those stories.
Even in original writing, take a moment and think about the different features of the character. Are you writing a well-rounded character or are you just focusing on bad parts so its clear they are the wrong choice? Are you trying to make a compelling villain or just make your other characters look better?
Consider your characters and your readers before writing a character simply to be the anti-choice.