Fiction with psychotic characters

I’m interested in fiction about someone, preferably the protagonist, who‘s psychotic. It can be any genre (I’d love it if someone knew of a fantasy book though) and doesn’t have to based on a true story. The psychotic character can be male or female or anything in between, young or old, kind or mean… it doesn’t matter. What matters is that I want to have a glimpse of how their mind works.

I read in English and in Dutch, so books in these languages are more than welcome.

Thanks in advance!