>>99760728Let the reader understand why the character is like that. Maybe they have bad life experiences that made them hate other people, maybe they've constructed a set of beliefs that help them justify their actions to themselves, maybe they were born emotionally fucked up and can't comprehend other people's feelings.
In a visual medium like comics, I think facial expressions are an important cue too. I remember a couple of cartoons where a cruel character showcased this bright, innocent, deceptively friendly smile as they were brutally murdering people. That clash between their cheerful demeanor and hostile actions was unsettling IMO. These characters looked like very young children that don't know any better, the sort of kids that might squash bugs for fun or pull a cat's tail. No understanding of morality, and no ability to consider the suffering of beings other than them. (I thought about posting a pic of the sort of expression I was talking about, but the examples that came to my mind are /a/, and they're also big spoiler reveals for their respective series.)
Also, try to avoid showing what the sadist villain did. Gory shock panels tend to get mocked for being edgy. It's common in horror movies to avoid showing the worst parts and leave that stuff up to the audience's interpretation. If people get hints about the horrible thing but can't see it, they'll imagine something far more scary than anything you could come up with.
