...and maybe Voldemort promised her to Snape in return for his undying loyalty. When Voldy (!) was unable to deliver (or perhaps just betrayed Snape), then Snape went back to the "Light Side" of the Force.
No one has yet mentioned the intense hatred Snape and Sirius Black have for each other. Maybe there was a love triangle there? (Poor Snape. How could he hope to compete with Sirius. sigh *swoon*)