Who do you blame?
Who do you blame the most for the death of James and Lily Potter? Voldemort, who performed the killing curse, Wormtail, who betrayed the Potter's hiding spot/location to Voldemort or Snape, who told Volfemort about the prophecy, which made Voldemort want to go after Harry in the first place?
I put them in this order:
1. Voldemort - well, he did kill them, and started the whole war and everything, right?
2. Wormtail -the bastard, if it wasn't for him James and Lily would still be alive.
3. Snape - even if Snape hadn't heard the prophecy, Voldemort would have gone after James and Lily when he got the chance, they were to powerful and to valuable to the order to be kept alive.