Why didn't Lily have to die?

[JoAnna]

No: HPFGUIDX 93445

Carol:
<<<What *I* don't understand is how he could have thought that *any*
mother would stand aside and let him kill her baby.>>>

JoAnna:

It's because Voldemort knows absolutely nothing about love, or the
power of love.  Dumbledore knows this, as he said in OotP:

"But I knew too where Voldemort was weak... You would be protected by
an ancient magic of which he knows, which he despises, and which he
has always, therefore, underestimated -- to his cost.  I am speaking,
of course, of the fact that your mother died to save you."  (page
833-34, U.S. edition)

"There is a room in the Department of Mysteries," interrupted
Dumbledore, "that is kept locked at all times.  It contains a force
that is at once more wonderful and more terrible than death, than
human intelligence, than forces of nature.  It is also, perhaps, the
most mysterious of the many subjects for study that reside there.  It
is the power held within that room that you possess is such quantities
and which Voldemort *has not at all*."  (page 843-44, U.S. edition)
(emphasis mine)

Of course it wouldn't occur to him that Lily would die to save her
child; he does not and cannot understand what power love can wield. 
I'm convinced that the tool Harry will use to vanquish Voldemort will
be connected to his love or his capacity for love, since this is a
recurring theme in the books (especially due to the emphasis that love
is the  power that Harry has - the "power the Dark Lord knows not"
spoken of in the prophecy).