Snapedom

Voldemort and Snape, along with Dumbledore, are characters given the "flashback" treatment in Rowlings' books, important enough to merit a backstory, but ultimately only a backdrop to Harry Potter's journey. Rowling reveals the bulk of their stories through passive devices, the Pensieve memory for the Slytherins and the tell-all book for the Gryffindor, thus limiting the character representations to her hero's somewhat unimaginative and unquestioning point of view. This means that for Voldemort, we are left with a psychopathic villain who has few dimensions. For Dumbledore, we are left with a complete reevaluation of his character in the last book. For Snape, for whom the author seemed to have little interest beyond use as a plot device in DH, we are left with many unanswered questions.

For example, we do not know how Snape was taken in by Voldemort. Snape warned that those who wear their hearts on their sleeves are easy prey for the Dark Lord. He did not say what sort of predator Voldemort was, however, or precisely how he became Voldemort's prey.