Chris Hallquist wrote: "Suppose Voldemort meant to do something else to Harry other than kill him, and that he succeeded in doing whatever he meant to do. If so, "your death here will not save your child" would turn out to be true. In that case, perhaps what amused Voldemort was realizing that Lily had misunderstood what he was going to do to Harry, and that she had offered up her life to prevent something that was not going to happen anyway. In that case, "Yourself to die, and the child to live" also reflects Voldemort's true intentions."

That makes sense. Maybe all he wanted was to turn Harry into a horcrux, and suceeded. Since creating horcruxes requires killing someone, killing James Potter was a convenient horcrux-energy source, while killing Lily could be an extra, but unnecessary bonus.