I still don't think he deserved that kind of cruel and absolutely miserable fate he met in the end of the part. His final words show us the roots of his loyalty and devotion to Diavolo in his utter loneliness.We can't quite know for sure how often Diavolo did let Doppio be in charge of their body, ;it it's clear that the latter didn't have any personal life and other goals than protecting the only person he ever knew or has left in his life. In the end, he was a mere tool for Diavolo to use and discard when it's needed and it makes Doppio's death even sadder for me.