Okayokayokay, we've already had discussions of a villainous Arty so I'm tempted to skip him cause I'm not sure if there's anything I can add. I do believe his 'villainy' would begin if Holly weren't able to save Butler in TEC. Or maybe it'd be later in the series, when he realizes that he hasn't been able to truly have a childhood, partly due to pressure from his family to be perfect and partly due to fairy shenanigans. It's true, Artemis was the one to involve himself in the People, but maybe this could lead to some self hatred as well, that's always fun. I also believe he would resent his family for shaping him into the person he is, not allowing him to truly be a child.
Ngl babes, I can't really envision a villain Butler, but the idea is intriguing. Maybe a Butler that said 'fuck this kid' and went off on his own? I can't imagine that either, Butler's too loyal. ¯\_(ツ)_/¯
OR maybe, with the disappearance of his beloved charge, Butler begins to resent the People? Hm...
YEAAAAAAAAAAAAAAAAAAAH GENOCIDAL HOLLY. I imagine a Holly who actually followed through with her vow to kill all humans for inadvertently killing her mother. Maybe she chose to join the LEP so she could learn how to handle weaponry and eventually get clearance to go above ground for genocide reasons. Perhaps being kidnapped by Artemis could be her first chance at genocide-ing. OR maybe she could PLAN to do the whole mass murder thing but growing closer to Artemis and co. changes her mind. This way, both Holly and Artemis have made each other better people by the end of the story.
What if, while Artemis and Holly were in Hybras for three years, Foaly started to feel doubts about the LEP. Numerous times they've tried to call off the searches, no matter how many times Foaly has told them that they will reappear. Without his best friend, Holly, and one of the only other people who can provide him with an actual challenge, Artemis, Foaly starts to... reconsider things. He really wants to marry Caballine, but he won't, not until he can have Holly there. Eventually, Foaly realizes that maybe the LEP doesn't deserve his genius. And what's the best way to rebel against police? Crime! Collecting blackmail via peeking into private conversations, tracking bank accounts, and planting bugs in offices.
And Caballine still loves him! He just can't stop commiting crimes. Tragic. OR villainous centaur lovers?