Why would Voldemort spare Lilly?
J.K. Rowling has said that while James Potter would have been killed by Voldemort no matter what, Voldemort gave Lily Potter quite a few opportunities to stand aside, thereby granting Lily the chance at life. I'm not asking why Lily didn't step aside because we know it's due to her being willing to die to save the Harry's life.
I'm asking why Voldemort gave Lily the opportunity to live at all.
It seems it would've been simple for him to blast his way through the Potters' house, kill James and Lily without a second thought, and then kill Harry without mercy. What was it about Lily that Voldemort even extended her the opportunity to live? He would have been left with a living witness to his attempted murder of Harry; Lily had already thrice defied him (James and Lily three times refused to go over to the dark side at his command); logistically, it would have been more practical for Voldemort to just blow through the Potters house and kill everyone in his path in order to quickly get to Harry.
I'd say offering Lily the chance to live goes against Voldemort's innate mercilessness. Wormtail had a moment of mercy and look what happened to him -- his silver hand from Voldemort strangled him to death. Presumably an act of mercy, like sparing Lily, would go against Voldemort's very nature.
So, why did he do it? Why did Voldemort offer Lily Potter several chances to live?
I'm absolutely not suggesting that Voldemort was secretly in love with Lily and that's why he offered her the chance to live. We all know Voldemort is incapable of love. Also, please no Wiki/Wikia answers. Regarding examples of Voldemort giving Lily numerous chances to save herself, see chapter 9, Grim Defeat and chapter 12, The Patronus, in Prisoner of Azkaban