Snape played both sides, because that is what Albus Dumbledore required of him....he killed Dumbledore because that was what was asked of him, by Dumbledore. Snape did not want to do it, but knew these plans had been laid out a long time ago between himself and Albus..he was not going to let Albus down. HBP was so obvious that Snape was indeed working with DD and doing what was asked of him. What about the hints that he yelled to Harry as he was running away? Snape had to run, because there was more work for him to do, work that Albus set out for him...if he were killed as well by the members of the Order...then the ultimate plans to kill Voldie may never have been carried out....Dumbledore needed Snape alive...to help Harry, which I fully believe that he will... The Unbreakable Vow was necessary for Snape to carry out, as Bellatrix was there watching....otherwise she would have run quickly to the Dark Lord. Snape was there for DD, when the Headmaster was injured securing the ring/horcrux....he saved his life. I believe at that time, that is when he asked Snape to kill him in the event that a student got in the way. DD may have even been slowly dying at that time. It is just a matter of a few months before everyone sees Snape as Albus saw him....loyal to his friend, and determined to be rid of the Dark Lord.