I think that Dr P. in
The Youth is probably my nastiest characters yet... but frankly, he wouldn't work without Christopher to offset him. I mean, you can't just have a nasty character and call it entertainment, you've got to have a nicer character to keep them running.
I actually don't like having characters that can be labelled as "nasty" and "nice" or "good and "bad". I think that's a clear sign of having one, unchanging character. They are human people, and human people only connect to characters that they can see themselves in. They can't do that if they don't have a little bit of a hazy area in their character.
I normally end up writing some background into a character, therefore giving readers a possible reason for their current characterization... but I suppose you could do it A Clockwork Orange style and give their actions no justification, no background, no reason. It tends to shock readers most... but it did get A Clockwork Orange banned for many years, for being too violent. Another way to do it is to give them one decision, or perhaps two, where they make the kindlier choice. Or perhaps they say something in a situation that they can't control which has a profound effect on the readers.
Then again, I don't like totally nice, understandable characters. I like to write them as people you think you know and understand. Readers start getting cocky and thinking "I know what they'll do". But they don't. Every character has one, colossal flaw, at least. Like... maybe they crack underpressure. Maybe they don't emphathize well with other people. Maybe they put people down too often.
My character Christopher isn't a totally perfect character at the moment anyway, but it's going to get worse in a way that can't be justified by what has happened to him in the past.
Anyway, don't worry about Dr. P. getting all mushy and understandable, there are worse characters yet to come in that story.