Please remember that just because someone is playing a villainous character doesn’t mean that they don’t love their character, or don’t have a version of it that is perhaps new to you. Respect the muns of villains as much as you would respect any other mun, and remember to work with them.
It is very hard to play a villainous character in a world full of heroes and cinnamon rolls, and the muns who pick them up and are willing to give them a try deserve some extra support and love, because they’re going to need it. Love the villain muns, respect their autonomy as players, and don’t try to force their character to submit to your plot desires just because you’re playing a hero. Heroes might not always win, and heroes are not always more interesting, not always better characters – they’re just typically more popular because most of us want to be good, and it’s a lot easier to self-insert with a morally good character for many people.
Respect your villains. If they’re canon characters, don’t assume that they’re all the same as every other one you’ve seen. If they’re original, respect that their mun is giving you a neat new antagonist to deal with.
Conflict is necessary, and villain muns deserve to enjoy their RP experience, too!