It was book one when he told Harry that Lily didn't need to die. And in book three, we heard through Harry's memories that not only did Voldemort not have to kill Lily, but he actually seemed to be trying to save her by telling her just to move away. By giving her a chance to flee and move on, Voldemort was showing that he didn't want her dead because given his M.O, he'd just shoot any burden in his path. And as you pointed out, Voldemort isn't exactly the first person to be trying to spare the life of a Muggle-born, especially when his entire mission is to "cleanse" the entire wizard race of them.
But that's as far as I'll go. I've learned far too well that JKR surprises her readers with explanations no one could ever guess. So if I wanted to speculate on Voldemort's motives, this post would turn into a novel because JKR could go quite a few ways on this.

Homer's words of wisdom on women: "If a woman says nothing's wrong, everything is wrong. And if a woman says everything is wrong, EVERYTHING is wrong. And if a woman says something is not funny, you better not laugh your ass off."