I must admit I always felt Snape was there when the Potters were killed but now I am not so sure. How would he have convinced Voldemort, after his return, that he was still loyal if he had gone to help Lily. I can't imagine he just stood there and watched her die.
I'm starting to move strongly in that direction, though. Re-reading Book 1, I find Harry having a dream that started with Malfoy's face, which morphed into Snape and a flash of green light and a hurting scar. The bright green light is almost a signature for Voldemort.
Speculation: Snape was there in Godric Hollow, and was instrumental in Harry surviving the Dark Lord's attack. He had to choose between saving Lily and saving Harry. If he was very fond of Lily, that's one reason he's so very resentful of Harry now. And it can help explain how Snape is so violent with "I AM NOT A COWARD!".
Slughorn proved that it was possible to modify memories against occlumency, and Snape is supposed to be the best in that arena. It could be that he allows Voldemort (and Harry) to see just what he wants them to see if they think they've broken through his barrier. I can't see LV trusting him so much if he had blocked all of his memories all of the time.
I still like the theory that it was Petunia who loved Snape, since Jo has already used the "mother's love" theme. Lily might have brought her potions buddy home to visit on a vacation. We don't know for sure that the visiting bloke had to be James. ..