Ok, Clock, just imagine a soldier/whoever the hell you can think about is gravely wounded after an explosion/whatever kind of hadicapping accident. He asks you to kill him, since he's suffering a lot, and his life is going to be something similar to hell if he survives. Basically, you say you won't have the decency to end his life, even if it's his wishes ?
I wanted to study biology because there's nothing more fascinating than life. Its pretended unicity through the while universe makes it even more mysterious, and make us happy to be something else than non animated materia. But if one of my relatives has an accident like that and asks me to end his days, I won't hesitate a second, even though it will be one of the hardest choice for me ever to kill someone I grew with.
By this, I'm basically saying you should stop focusing on what YOU believe about this, and actually listen the one that is actually more concerned by his own death.