[Lex79]

I assume that the market for stories with a protagonist that is irredeemably evil and completely unsympathetic is much smaller than the one for stories with an evil protagonist you can still relate to, and probably publishers also push in this direction so they don't gain too much bad press. Same reason why in almost hentai I read where the protagonist forces himself on a girl it turns out she already loved him so in the end it was OK.
The series with the most evil protagonist I ever read is probably Maou no hajimekata where the protagonist does objectively evil things to good people too but even there the author gave him some attenuating circumstances so readers can relate to him.
Regarding slavery, in all the series I read it's either something bad that the protagonist has to fight or an excuse to collect girls and I never expect anything different when I approach a new work.
All of this works for me because I prefer stories with a protagonist that is good or at least has so,e chances of redemption, but I can see how someone looking for stories about truly evil guys may feel frustrated. Maybe it's easier to find such stories among web novels, assuming the language isn't an unsurmountable obstacle.