Well, first off, spoilers ahead, as they say on the internet. If you are one of the three remaining people who have not read the Harry Potter series, but plan to, then don't read this.
I have written before that the Harry Potter books are occasionally marred by the presence of gimmicks and hand waving to move the stories along. And yet, there are parts of the books that are written with great authenticity and subtlety, and the character of Severus Snape is one of those high points. Throughout the first six books, the motivations and characteristics of Snape are called into question, with hints and red herrings being brought up in every book as to whether Snape is a double agent, a triple agent, or something else entirely. Snape has a pathological dislike for Harry and his friends, has admitted to having practiced the Dark Arts and being a follower of Lord Voldemort, and generally being unpleasant and angry. And yet Albus Dumbledore, the wisest of the wizards, trusts him implicitly, and states so over and over. Up until the end of the sixth book, when Snape kills Dumbledore, seemingly in cold blood.
And just when the mystery is closed, it is reopened up in the final chapters of the last book: Snape was not evil at all, but was a deep cover agent working for Dumbledore, who killed Dumbledore on his own orders to preserve his cover, and because Dumbledore was doomed by a curse to die soon. And Snape's motivation for rebelling against Voldemort was a long-burning love for Harry's mother---whose death he had inadvertently caused, giving him a mixture of fierce loyalty to the downfall of Voldemort, and a good amount of self-loathing, as well as paradoxically placed loathing for Harry. All of which, if it doesn't make sense here, makes sense after reading the books, and in retrospect. While this sounds like a cavalscade of information in summary, it is all subtly and artfully laid out in the books.
In addition to being a tortured soul, there is another important fact about Snape, and the reason that I held to him being good all along, was that Snape was really smart. This is an important point, because while most of the Death Eaters were motivated by a desire for wealth, power, or the ability to indulge their sadism, Snape's interest in the Dark Arts seems to be a matter of academic interest. When his home life is shown, he is revealed to live in a cramped cottage filled to the ceiling with books, a fact that endeared him to me. Since Snape's academic and intellectual nature seem to be such an important point, I didn't believe he could truly be devoted to the mixture of thugs and psychotics that made up the Death Eaters.