We see Cotillion stabbing Kaminsod in the back, which I think WAS a good thing? Does he go back to wherever he came from?
It's left uncertain.
As far as I know, it's left vague on purpose. It was stated somewhere in The Crippled God that Kaminsod's current body was unfit for travel back to his world which leaves me to believe that Cotillion acted in a way that would free Kaminsod from his body, allowing his soul to travel back to his world without issue.
We probably won't know for certain unless future works clarify or via the word of Erikson. Due to a lack of evidence to the contrary, we're left to believe that's the intention behind Cotillion's actions. Just as the characters however, we're left in the dark as to whether it succeeded or not.