If we do not have control over the most basic aspect of our lives, or own death, can we ever truly be free? For some people life itself is a prison unlike any other, something the only escape from is death.
Not only do I support a person's right to their own body and life, I believe that trying to keep a person who is truly suffering from ending their suffering is simple greed.
If your dog had cancer and was dying a slow and painful, and possibly a quite humiliating death as well, would keeping them around be considered "saving" them, or would it be cruel? Respect for life has to respect all aspects of life.