I understand what you meaning. and i know other can have different opinion then me.
But. 12 years ago i lost my dad from Cancer and i know the feeling of losing someone, and yes he had a lot of pain and was using morphine i think it was to ease the pain.
Still i could never have stopped his life even if he begged me. for me killing is not an option.
I know exactly what you mean. That was your Father. It would be hard for me too, I don't even kill a spider, much to the disliking of my Sisters, lol, if I can avoid it. And I am very sorry for your loss, I didn't know my father at all, and when I had finally met him, he was gasping for breath every few seconds, with tubes all down his throat, he had multiple cancers throughout his entire body, and a couple of days later, he died. However, what I had proposed, or for most cases, the person suffering, themselves, would 'pull the trigger' persay, unless they cannot move.