Of course. If you have a dying animal, you'd be arrested for animal cruelty if you don't put it out of its misery. Yet for some reason some people insist that humans in that position should be forced to continue suffering. I won't even pretend to understand what goes on in the twisted mind of someone who would wish further suffering on the sick and dying.
This one. If you have a cat and is told that it have a disease that would take a year to progress that would cause it pain, would you let it? That would be selfishness on your part.
I guess the same can be said of humans. It's about the quality of life lived. I'd rather die now if I know that I'll be spending the remaining years of my life in coma with very little chance of waking up again.