A wild shybug appears, did someone say my name?
If someone is in a lot of pain, and is going to die from a incurable illness (ie cancer) at the point that there is no hope, I have no problem with them choosing to die or not, as there going to die anyway, you're just making them deal with the pain longer.
Personally, I find it the most humane thing you can do, under certain circumstances, is let them choose.
I'm going to leave this hypothetical question, if a mouse is trapped in a mouse trap, and it's suffering and alive with a broken neck, would you kill it? or would you let it wriggle around in pain for the next 5 minutes, both are fucked up, but what is the most humane thing to do in that case? personally I would put it out of it's misery. The mouse will die anyway.