you have just summarized the problem of pain.
Why does God give us pain?
There are no easy answers in any religion. But suffering exists.
So what would you do about it?
and if you start saying “kill everyone who is suffering” to stop their pain, you will end up killing a lot of people.
or you could dedicate your life to helping the suffering, maybe by just helping those around you with little things.
Of course you have to take it on a case by case basis. This person's life is already over with. There's nothing left; just a one-time person waiting to die and suffering in the process. He can't speak, see, or do anything. He is already dead for all practical purposes. His continued suffering only benefits one entity; the hospital or paid care givers of his shell to keep his body alive so they can continue to get paid a crazy amount of money. If he could speak; I bet he would ask for an overdose of morphine or other drug so it would be like going to sleep then never wake up on this plane again. His afterlife will give him what he craves. That is if he even has enough brain power left to understand his situation.
No one is talking about "killing everyone in pain". I have no idea where you got that idea.