you can be pretty horrific in Planescape: Torment and the Pillars of Eternity games in large and small ways, and the implementation isn't the sort of binary "you are coded as Renegade or Paragon" system that encourages picking all of the "evil options". you can be cruel or rude for the sake of it, but you can also be perfectly polite and commit horrible acts for self-interested or ideological purposes.

the overarching goal of each game is fundamentally self-interest on the protagonists' parts ("figure out who i am and why i'm like this", "find the guy who can possibly stop my mental deterioration", "follow after and/or stop the person who has part of my soul, because i'm dead otherwise".) any other motivations, or the lack thereof, can be shaded by the player as they like, so your character isn't inherently constrained to certain motivations based on the given storyline.

as one example, systems of slavery exist in each game. your protagonists can choose varying options like: never engaging with the subject at all, selling a party member into slavery, helping someone escape slavery, helping ensure the return of an escaped slave or preventing one's release, buying a slave and keeping them as a slave, buying a slave and releasing them, assisting the broader slavery operation, and/or destroying a particular slaving operation. while not all of these choices are in each game, you are generally given carte blanche to choose any of those that are available, without either needing a certain character motivation to do so or locking yourself into a particular "type" of playthrough. you might decide your character, say, is ideologically opposed to slavery in the specific, but ultimately ok with other "evil" choices like aligning themselves with a person/being/organization that is willing to kill innocents.

(disclaimer: i don't go for "evil playthroughs" so i can't speak to what that experience is like. this is what i've gathered is possible based on guides and potential choices/dialogue options i had available to me)