Sure, Theon has done some pretty despicable things in his time, but I’d say that castration and torture at the hands of Ramsay Bolton taught him his lesson.
Alfie Allen has already teased a few things about his character’s path in Season 7, so we know that Theon won’t die immediately.
Now that the Boltons have been broken and the future of the Iron Islands seems to be in his sister’s hands, there may not be much more for Theon to do other than die.
That said, there are still plenty of people who would be more than willing to kill him, and the poor guy probably wouldn’t even protest all that much.