In my original version of this, I wrote "Write them right."
Oh, young me. You have a lot to learn. Luckily, current me is a little wiser and open minded.
When writing canon characters, it's very important to write them in character. Don't have them go off and do outrageous things that the character would never do. As much as you may dislike a character, don't write them as an absolute asshole unless that's the point of the story. What I mean by that is, don't go write a fic where Spottedleaf goes around acting like an utter bitch just because you don't like her. However, you could write an au where Spottedleaf is a villain. That being said, I don't think she'd go around being a bitch even then! This is what I talked about in my last version of this, and I think that still stands.
However.
People have different interpretations of characters. One person may absolutely love a character, others will hate them. Someone may believe a character would do a certain action, another might say that's out of character. People have different views and interpretations. For example, I know many people who ship Jayfeather and Briarlight, while I myself can't envision the two having any form of relationship other than platonic. That doesn't mean that any fic featuring BriarJay is out of character, it just means that some people see that particular relationship differently than I do. That's okay. No one is wrong here.
Same goes with actions, as mentioned before. If I wrote a fic with Firestar breaking a rule, some people might say "no, Firestar wouldn't do that, after becoming a leader, he became much more serious and responsible," and another person might argue back "actually, Firestar never lost that sense of fun, he just wasn't a protagonist so we didn't see much of him." Both of these could be correct, because there's nothing in canon that could suggest otherwise.
So while I wouldn't suggest writing a "actually, Bluestar attacks the protagonist because I don't like her" in your fic, I also think there is some ways to get around that. If you really hate Bluestar, write a fanfiction from Thistleclaw's perspective. We all know she isn't really the villain, but from that specific point of view, she is one. If you desperately want to write about your female-oc who was actually Firestar's first love, not Spottedleaf, being attacked by Bluestar because you didn't like her, make it during the time when Bluestar was overtaken by paranoia and believed her whole Clan to be traitors. That way, it's that little bit more realistic. Because no matter if you dislike Bluestar and perceive her actions to be selfish, or if you like her and perceive her actions to be shitty but driven by grief, or whatever you feel towards her, you have to admit that logically, Bluestar isn't going to just attack a random cat in her Clan.
But it's your fanfiction. Your au. You can do absolutely whatever you want. Trust me, I'm in the planning stages of writing an au where Dovewing, Hollyleaf, Jayfeather, and Lionblaze are all in different Clans, and they're part of "The Four." And the timelines are pretty different. That's okay though! It's an au for a reason.
In the end, try and write your characters as in-character as you can. Try to look past your own emotions and analyze the character. Don't be afraid to add depth, especially when they're one of the less developed characters in Warriors, like our lovely Lionblaze with the personality of a piece of soggy cardboard. Inevitably, people are going to disagree with certain portrayals and that's okay. Just have fun with your fanfictions, and if you intend on being a serious writer in the future, or even now, try to take them as seriously as you can, but still have fun with it!