May I use religious lecture to anwser your question? If any of you is uncomfortable about christianity, lemme beforehand apologize for this to come:
When our Lord was nailed at the cross, after three days of suffering, Him, knowing that death was inevitable and all that was happening now was him extending his suffering, begged for the release of death. And a roman soldier, by the name of Longinus, in an act of mercy, stabbed Jesus with his spear, killing him. This man, who had just commited the sin of murder, and of out Messiah no less, was not doomed to Eternal Damnation for his actions... he was risen to become a Saint.
An example of how, when one knows that the only thing that lies ahead of you is suffering until the release of death... cutting it short is completely justifiable. Even noble, as that who does such merciful act for you may become a Saint.