I have been wondering about this for a while. Why was Voldemort about to spare Lily? It doesn't make sense. Voldemort, who values blood above all else, kills Pureblood James, but gives Lily the opportunity to live. This, of course, must have to do with something we have yet to find out about Lily.
I killed your father first, and he put up a corageous fight...but you mother needn't have died...she was trying to protect you (SS).
Stand aside. Stand aside, girl! (PoA)
We really don't know much about Lily. She was very good at Charms, which was mentioned in SS, but then, we find out she was very gifted in Potions also, in Half-Blood Prince. Snape was also very good in Potions. She was very liked in Hogwarts, especially by James Potter and Prof. Slughorn. And she could have lived.
Voldemort has no mercy. He doesn't care about killing people, or who he steps on to get to the top. But something made him decide to spare Lily, which is very out of character for him. What do you think could have made him decide to let her step aside?
His eyes are as green as a fresh pickled toad,
His hair is as dark as a blackboard.
I wish he was mine, he's really divine,
The hero who conquered the Dark Lord.
--Chamber of Secrets
"But we're not stupid- we know we're called Gred and Feorge."
"'But Dumbledore says he doesn't care what they do as long as they don't take him off the Chocolate Frog Cards,' said Bill, grinning."
"I s'pose Lord Voldemort's just a warm-up act compared to you three."