I admit I had my doubts, but Snape always seem to be in the right place at the right time good and bad. In the end we all seen where his loyalities were and that was with Dumbledore and Lily. He loved Lily for all of those years without his love being returned. But he had been bitten by the love bug and couldn't help himself. And what did Dumbledore say to Harry....that LOVE was the most powerful magic and Voldamort would never have it. Well I think when Snape realized that he LOVED Lily he choose his path and knew it wasn't going to be easy.