"If she means so much to you," said Dumbledore, "surely Lord Voldemort will spare her? Could you not ask for mercy for the mother, in exchange for the son?"
"I have — I have asked him —"
"You disgust me," said Dumbledore, and Harry had never heard so much contempt in his voice. Snape seemed to shrink a little. "You do not care, then, about the deaths of her husband and child? They can die, as long as you have what you want?"
Snape looked up at him incredulously. "You think I could dissuade the Dark Lord from killing the child he thinks is destined to kill him? He would kill me even for asking - think the request alone was evidence of treachery - and then no one would be saved!" His voice rose. "Do you know how I had to beg him to even spare her? He grew suspicious - wondered at my game - only by convincing him I asked only from the basest reasons did he agree to my request - much less allow me to escape alive!"
"And what of her husband?"A shocked laugh broke from Snape. "Why would I ask for James Potter's life? Even if I wanted to-" He shook his head, his stringy hair flying about his face. "How would I? Do you not understand what a great boon I have been given - for even one Muggle-born who has personally defied the Dark Lord thrice to be allowed to live, even in captivity, even in what he believes will be utmost degradation? The Dark Lord's philosophies are completely opposite to yours, Dumbledore, he thinks even the most horrific life is a thousand times better than death! Now you wonder why I did not push my luck - why I did not beg for the life of a man I despise? Why him? The Dark Lord has killed far worthier men than James Potter!"
Author's Note: I have honestly never understood why Dumbledore grandstands on this point. James is a valid point, because Snape probably wouldn't care about his death. But how could he get away with asking for Harry's life? "Milord, about that prophecy I brought you, about the child with the power to vanquish you... Er, would you mind sparing that child? What? No, not for a day. For the rest of his natural l- Yes, that would involve him growing to the age where he could point the right end of his wand at you, and all sorts of other inconvenient things. That's absolutely just a coincidence, th- Milord? Milord, please lower your wan- AAAAAAAAGH!"
The bizarre thing is that Dumbledore could easily have hit Snape on a very similar point - something like 'You accepted all of Lord Voldemort's other murders, but the moment he threatened someone near to your heart, you came running to me?' Instead he specifically complains about the one person Snape would never plead to have spared and the one person he could never plead to have spared. I mean, maybe out-of-universe people's lives only matter based upon their proximity to Harry, but surely that's not the case in-universe... right...?