Having had near death experiences, I can say that it was a sudden change for me. Also, there was plenty of foreshadowing toward Rust's change in the previous episode.

My opinion is that the show ended perfectly, as it was the entire time.

People who wanted more of a twist weren't watching the right show. I guess the title "True Detective" might lead one to believe such a thing, but the show has always been about the development of characters, not about the mystery.