To me, that's precisely why a redemption arc for Cassie would be so intriguing. Unlike most such arcs, Cassie has a lot to atone for. She didn't make a regrettable mistake for good reasons, she deliberately did horrible things for the worst possible reasons. She's thoroughly earned her comeuppance, and redeeming her would cheat us of that.
Of course, if redemption were limited to people who were 'worthy' of it, it would be a meaningless concept. Cassie is an evil person, but she can change that if she wants to. It wouldn't undo the terrible things she did, it just means that she accepts she was in the wrong and resolves to do better going forward. She'd probably do what she could to mitigate the consequences of her old actions, but her victims are under no obligation to thank her for that; it is quite literally the least she can do. Seeing Cassie become a better person wouldn't be as viscerally satisfying as rubbing her nose in her crushing defeat, but in the end it is the ultimate indictment of what she once was.
I still say such an arc can only work if Artemis is going to be a very long game, though. This can't be something that the MC brings about by racking up Love Points with her. She doesn't 'earn' a redemption by becoming sympathetic; it's the other way around. This is something Cassie has to do her own because she's come face to face with her own sins and flinched. The MC can participate in the process if we want, but it can't be about him validating his old flame. The focus has to be on Cassie being overwhelmed by genuine remorse for all the people she's hurt.
We're nowhere near that at the moment. Most obviously, Cassie is currently impervious to empathy. That needs to change. Realistically, she would need to face a serious revelation, something that can shake her to her very core. It will take a lot of time to setup something like that. Once Cassie hits rock bottom, it will take more time for her to recognize the possibility of change (this is where the MC can get involved, if we're so inclined). It will take even more time for her to commit to the new path and alter her ways, and then still more time to show the final result.
That's a hell of a lot of time and effort to spend on a side character, especially when most of us will be eager to skip the arc entirely and tear her down as far as we can. I'm not sure if it's realistically possible, but I can't help but respect the ambition if that is the goal. If, however, Cassie would only have time for a short, trite redemption arc, then it's better to leave her an unrepentant villain and let us revel in taking her down once and for all.
