I mean... she has a point though. You've stumbled into a region that's far beyond saving even before she showed up. She's the only one upfront about her evil. "Yeah I corrupt and kill, I know that makes me bad. But you know what? Here's my reason bro. Don't like it then oh well." Everyone else hides behind excuses and religion. Like I said, there's literal childgrooming and probably classes. Take that as you will.
In a world where people justify them eating people's essence, childgrooming, slavery (sans pleasure), etc why is Kas so evil?
The real world has/had literally all those things at a similar level of technology....except worse.
Eating peoples essence isn't really a huge deal....they specifically don't kill people, and it seems to be more of a minor inconvenience than anything.
Slavery is everywhere is any society in this time period, I don't see how it makes this world particularly corrupt.
And the child grooming thing is...well, nothing. Again, if we're comparing this to the real world? We would have those kids doing the same thing, except we didn't really do the "wait until 18" part. Heck, most of the excesses in this game, besides the glut of sex which nobody hugely minds, us sanitized compared to IRL history.
Kas wants a world where alraunes, corrupted bees, demons and goblin succubi are roaming the land. Kas wants a world where she doesn't stop at taking your essence, she drains your whole soul. In her world, kids won't be "groomed", because I doubt all the demons and whatnot will care about a kids age at all before they get started.
Literally everything Kas has done is a worse version of stuff already going on. And that's not counting the breeding experiments, the cult (which she doesn't directly control but is certainly the cause of), what she did to the Winter City (they didn't dwell on this, but the entire government and royal family of WC is dead now), or the fact that she's doing experiments that are ripping open the fabric of reality and letting in extra dimensional soul stealing monsters, which if Lumia hadn't been around for would have probably completely ended the Frost Marches and probably the world right there....
Besides the loads of casual sex, the world says Kass is pretty much just normal feudal society plus magic. There's really no comparison, Kas us going to burn the whole thing down.