LV and Snape - Ubreakable Vow

[B.G.]

No: HPFGUIDX 134795

Perhaps LV and Snape made an UV that LV would not kill Lilly.  Why?  
Maybe to reward Snape for a job well done (prophecy).  LV had no use 
for Lily and didn't care if she lived or died so no skin off his 
nose.  BUT in anger when she wouldn't move from Harry he killed her 
and as the AK bounded off of Harry and the broken UV did its magic - 
LV's body was destroyed (skin from his nose and all).
Whatcha think?
-BG-