For anyone who doesn't know, I'm 99% sure that the idea for Kalysea is stolen (in a good way) from Mystic Knight Maya. There's a bad end in that game with a futa centaur, where Maya sucks the centaur dick until the centaur's magic cock ring binds her to the centaur... Really makes me wish that Wsan was writing the bad ends for that game!
Also, Kalysea is more morally grey than evil, imo. Her motivations to protect nature and fight demons seem to be genuine but, yeah, magically drugging people into being your forever cockslave isn't exactly a virtuous thing to do.
ETA: it's also not clear that there's no choice behind becoming enslaved to Kalysea. If the champion wishes, it's possible to avoid Kalysea's control entirely, or even just to have a normal sub/dom relationship with her. You never fight her and she never pursues you if you don't want her to. That seems to imply that the champion wants the life of a cockslave to some degree.
This is further enforced by the fact that even after she has the champion powering up her magic, she doesn't go back to the champion's camp and bind Cait, Ryn, etc. to her. I think if Kalysea were true evil, she'd be hunting people or manipulating them more. Instead, she kind of just knows that she has a supernatural influence and doesn't stop people from binding themselves to her. Still bad, but maybe not evil?