The Warrior Cats series is generally pretty clear-cut as to its characters' morality, which is good news for those who like villainous characters. Who are your favourites in the series? :)
I really enjoyed Brokenstar as a villain; I find him to be very understated in the fandom, probably because he only showed up in a few books, but compared with a lot of the more ineffectual villains (e.g. Hawkfrost, Ashfur) I found him to be a much more interesting character because he actually achieved things -- such as driving an entire Clan out of the forest. I feel like a lot of the later antagonists were too quickly dismissed from the stories, which detracted from their villainy. I also remember getting very emotional during the scene where Yellowfang killed him with deathberries, as it showed that despite his presentation as this oppressive dictator, he was still someone's family, which I feel made him more realistic as a character.
I enjoyed the plot with Mudclaw's rebellion as well; I found him to be one of the more justified villains, as he genuinely felt he could do what was best for his Clan, despite his rather harsher nature than Onewhisker. I feel like I should read the book with him in it again at some point (Dawn or Starlight, I think?) to see whether my perspective is still the same, as it's been a long time since I read any of the New Prophecy books.