I can't speak for all writers, but it seems that for me, things that scare me or things that I'm just plain uncomfortable with — people, patterns of behavior, creepy situations — end up in my writing.
Google "shadows Carl Jung" and you'll find a theory that the things that disgust or horrify you the most are the things that you are trying to fight in yourself, or the things that you find worst about yourself. I definitely find some truth in this theory.
Please refrain from diagnosing antagonists and/or protagonists (really, any character) with personality disorders (such as borderline, histrionic, antisocial). Or at least if you can’t resist, please don’t post it in the comment section. Instead, try to think of my characters as they are — flawed, possibly very mentally disturbed, human beings.
When I was writing Not Just a K-Girl, I actually found it kind of...entertaining...coming up with all the lies and ways that Kaden would try to (and often succeed) to get under Paris's and other character's skin. In fact, in one of the chapters that is at once my favorite and least favorite (if that makes any sense), my MC is probably the most deluded/stupid/manipulated she is in the entire book. AND IT WAS REALLY FUN TO WRITE. Sue me.
Kaden: WHAT?! I asked Chloe to tell you two weeks ago, and ask if you wanted to open for me! She said she had, and you’d said no!
I put down the phone, boiling. So much for Chloe being concerned about me! Showed what a friend she was!
Just shut up Kaden, and go die in a hole.
Someone else in reply to first reader: Personally, I think holes are too good for him. A sewer is more my thinking...