***Spoiler warning! If you are not a Harry Potter fan or haven't read all the books yet, you might want to skip this post.***
I was watching an interesting debate about Severus Snape and whether he is a friend or foe. Both sides made some good points, although I felt the "Snape is a Friend" side had better arguments overall. Frankly, I think they needed a "Snape is only looking out for Snape" side.
One point I found lacking in the arguments was that it appeared that no one considered Snape's last words to Harry as he ran off to join the Deatheaters.
"Kill me then," panted Harry, who felt no fear at all, but only rage and contempt. "Kill me like you killed [Dumbledore], you coward--"
"DON'T--" screamed Snape, and his face was suddenly demented, inhuman, as though he was in as much pain as the yelping, howling dog stuck in the burning house behind them--"CALL ME COWARD!"
This is how I consider this. From reading the books, it is clear that Dumbledore only shares information with the people who need it. If you are living in a world surrounded by your enemies, the fewer people who know what you're up to, the easier it is to carry out your plans. Snape's job is most certainly to infiltrate the Deatheaters, and to prove his loyalty it was necessary to make an unbreakable vow. If he was working for Dumbledore, he returned and told Dumbledore that he made a vow to protect Malfoy and carry out Malfoy's mission should the boy fail. Dumbledore is not selfish. He does what he does for the greater good. If he recognized that Malfoy's mission was to kill him, he would have insisted that Snape carry out that mission, should it become necessary.
Dumbledore was the only person in Snape's life, on either side, that trusted him completely. He forgave Snape for his past mistakes. I don't think Snape would have taken that lightly. His anger when Harry calls him a coward in the scene above is because Snape is the only one who really knew what was happening up in the tower. He showed incredible courage to do what Dumbledore asked him to do, but he is the only one who knows that. I am certain that this was done so that Harry will be able to carry out his mission--killing the evil Lord Voldemort. Let's face it, there's no way Harry will be able to accomplish that feat without someone on the inside to take Voldemort by surprise. Snape will provide Harry the distraction that he needs to complete his mission and probably die in the process, but he will be vindicated in the end.
Of course, this is just my own theory. No one will know for sure until next week!