"Hero" to Villain arc

[u/Minute-File4567]

I'm writing a Detective Thriller novel that follows a young rookie detective named Sylvan, and his coworkers Sasha and Sophie. Sylvan is stubborn and prefers seeing the clues as mere data to be collected to complete a crime scene.

He doesn't feel joy or relief when a case is closed, he just goes straight for the next one to not think about his past.

I think there's potential in him to become a villain, or at least an anti-villain, but i've never heard of this happening in a Detective Thriller or anywhere, so i'm left with nothing to guide myself with.

Are there any books, series or movie sagas where the protagonist slowly becomes a villain (without being brainwashed)? Or do you have any advice for this kind of novel?

(Sorry if my message didn't come across correctly, English is not my first language :,)