Friend or foe?
I'm voting FOE.
I think he has irrevocably put himself on Voldemort's side by killing Dumbledore. That doesn't mean he's 100% evil. Obviously, the man has some good in him, enough to garner Dumbledore's trust. He was abused as a child and can deserve some sympathy. He may have been like Draco Malfoy--playing with the dark arts but not really, seriously cut-out for the job. But he enjoyed killing Dumbledore, and in my book, that puts him on the dark side, FOE.
And I'm amazed at how many people, serious HP fans, truly believe Snape is good, or at least not evil/foe material.
Watch this discussion: PhoenixRisingConference