When writing a novel, what is your advice when writing chapters of a dark nature?

If everyone was afraid of “going too far”, we wouldn’t have James Elroy. You are describing a villain so it’s necessary that you show your reader what is in his mind, especially when you want your readers to hate him. The evil thought helps sharpen your character’s personality. But you need to make it convincing. Like, you can go as dark as you want, but give your character a reason for all his doings. Something that will justify his action. Your readers don’t have to agree with him, but it’s reasonable to him.

That’s just a general advice. I don’t know exactly what you want to write.

While describing the actions of the villain, you can be pejorative in descriptions of his temperament.

“Corrupted by the poison of his soul, he took pleasure in the pain he caused as he bound her head back and flexed her back.”

Write your story and damn who doesn’t like it. Stephen King never apologized. He said; “If I can’t terrify you, I’ll horrify you. If I can’t horrify you, I’ll go for the gross out. I not proud.”. Honestly. Do you think William R Burroughs worried about what people would think? Or any writer, for that matter. Writers write. Period. If you worry about what people think you’ll never get anywhere. There will always be someone who feels it doesn’t suit them. Forget about seeking approval and just write. And remember, bad guys don’t think they’re bad.

Create a backstory to explain why the person is the way they are. Also remember that villains typically don’t see themselves as villains—they see themselves as heroes (it’s just their mortal compass no longer points north). If you look at some of the worst villains in the world—child molester, you’ll find all kinds of nonsensical justifications they use in their own heads to justify being monsters. The Nazis thought they were improving the human species; Chairman Mao’s Cultural revolution was about feeding the Chinese people; there are a lot of douchebags on Wall Street who justify their actions via Ayn Rand’s crappy philosophy, etc.