Why Lily didn't have to die.

[eggplant107]

No: HPFGUIDX 137617

We know that Lilly didn't go out with James until the 7th year so in
the 6th she must have been going out with somebody else. Snape and
Lilly must have been in the same potions class in their 6th and the
term "Half Blood Prince" sounds like a pet nickname a girl might give
her boyfriend. Perhaps Lilly was the real potions genius and Snape was
flunking out so Lilly wrote all that stuff in Snape's book to help
him, Hermione did say it looked like a girl's writing and the notes
never said "I am the Half Blood Prince" it said "This book is the
property of the Half Blood Prince". And perhaps that's why Lilly
didn't have to die, 
Voldemort knew Snape still had feelings for Lilly so as a favor to his
right hand man he was going to let her live, but she just wouldn't
stop protecting Harry.  

Eggplant