I think that Snape either never left the Death Eaters, or he returned to them after Voldemort's return (like he said to Narcissa and Bellatrix at the beginning of HBP), or he is, and this is what I really think, simply torn, confused, and afraid. He doesn't know what to do. There is a theory that Dumbledore's death was part of a plan to give Harry the necessary jolt, sort of a good swift kick to set him in motion. I think he's really dead, but it would be interesting to see what his portrait in his old office would have to say about the whole situation.