I say, it all depends.
If we are talking about a sick, suffering animal who cannot be helped in
any other way, then it is mercy. It is the last, and the hardest, loving
act we do for a cherished pet.
But when it comes to a human being, then it is murder. Yes, it is hard, very hard, to watch a loved one suffer. When my Baba was dying from stomach cancer, I prayed that God would take her, and when He did, I thanked Him for ending her suffering.
Yes, I thanked Him, but He was the only One Who had the RIGHT to end that suffering.
I would never have done anything to hasten Baba's death, even by an hour. Even by a minute. Even by a second.