Book series where the main hero in book 1 becomes the main villain in last book

I don't mean secretly a villain all this time like in the star wars.
I mean someone genuinely a hero in book 1, turns into a villain throughout the series, becoming the main villain in the last book.

I've read Baru Cormarant's first book. It felt like the series was heading in this direction. But later books are too long for my preference, so never read them.

No romance, no young adult. Preferably fantasy, but sci-fi is good too