... Actually Evil loses because ultimately whether or not they're good at the whole "ruler" thing 'heroes' eventually step forward to try to kill them.
And in truth, the Second Overlord died heroically, to prevent the Athe from escaping the Hells where a different god banished him and forced everyone to forget his name, even himself.
The Third... Well, till they make an Overlord III we won't know how, or if, he was defeated or if he just went "... Fuck this. I quit." and just gave someone else the job.
Both Good and Evil are self destructive, just in different ways. Good destroys itself by constantly stabbing itself in the back and fighting amoungst itself. Evil self-destructs in that generally speaking evil people are a touch more selfish than strictly desirable.
Henriette is a necromancer, Jess is a half-incubus who is fully on the side of evil, Catt is a vampire who loves to eat people and basically anything else filled with blood. Everyone else on her side is minions.
Her accomplices are far and away from Good.
... I'd accuse you of racism here, but it'd be WAY too easy.
Frankly saying "EVIL... because Necromancy/Birth/Curse" is just... stupid. Necromancy being the only one you can say someone is evil on that basis alone because it's an active choice. Even then, it is very rarely what you're using, so much as how you use it. And you seem to have neglected to mention that the other half of Jess is a major Good individual as well. Or at least, is regarded as such. Cattleya has, as I already noted, only really killed people who tried to kill her... or were themselves evil. It's not like she killed random joe number 5 in the town of Niceville (which isn't corrupt unlike half the other places they've been too so far).
The minions meanwhile are essentially only as evil as the one directing them, which is the Overlord, not Gnarl (no matter how much that had to have annoyed him in Overlord 1) and the current Overlord(Well, Overlady) has, so far, not done anything that can actually be looked at as Evil (except by people actively looking for Evilness).
Gnarl is her one associate who is undeniable, and totally, Evil in and of himself. At the same time he's also slavishly devoted to the one who holds the title Overlord and really can't be seen as "Proof" she's Evil, or performing Evil.