Why not kill Lily?

[ginny343]

No: HPFGUIDX 142834

I'm wondering about the statement Voldemort made to Harry about, "Your 
mother needn't have died".  That seems out-of-character for Voldemort -
 seeing as one of his big things is pure bloodedness (even though he 
is not).  He has obviously killed a large number of people, why on 
earth would he decide to kill James, little Harry, and spare muggle-
born Lily?  Some time ago I read various theories about this, but I 
can't imagine any reason why Voldemort would intentionally consider 
not killing Lily.  Any ideas?

Ginny343