Others have given you an angle related to consent. I'll give a different take: Rehabilitation. In some philosophies, Prison is designed to rehabilitate prison

Others have given you an angle related to consent. I'll give a different take: Rehabilitation. In some philosophies, Prison is designed to rehabilitate prisoners so they don't reoffend. Execution doesn't fit with this philosophy. Assisted Dying is meant to be an option for those for whom there can be no rehabilitation. Eventually some day, when we've figured out the key to immortality and/or sufficiently advanced robotics, maybe we can offer a different way to rehabilitate people facing terminal illnesses and suffering. Until then, letting someone choose not to suffer out their final years and die on their own terms seems like a reasonable approach.