Why didn't Lily have to die?

[greatelderone]

No: HPFGUIDX 93384

Just out of curiosity, but why needn't Lily have died on the night 
when Voldemort attacks the Potter residence. Voldemort himself 
mentions it first in PS/SS and then later in POA we hear him tell 
Lily to stand aside. Seeing how he killed James, a pureblood though a 
traitor, why does a ruthless Dark Lord actually show an ounce of 
mercy to Lily who he considers a mudblood according to his ideology 
and who probably defied him thrice according to the terms of the 
prophecy. I mean he could just have systemically slaughtered the 
whole potter family. Why did he actually try and make an effort in 
having her stand aside? It's so not like him imo.