If someone is in a coma and pulling the plug would take days to kill them, they should euthanize them. Either way the person will die, but euthanasia speeds up the process and helps if they are still conscious in there somewhere.
It's a weird place where we don't consider it killing to let a helpless body starve, but we do consider it killing to inject something that ends things quickly. Both sound like killing to me, and both are ok in some situations.