My speculation is that Severus Snape is not, in fact, a traitor to Dumbledore and the Order of the Phoenix.
My first observation is that when Severus is answering all of Bellatrix's rude questions, he very much avoids answering why he hasn't killed Harry. Further more, Severus dances around any real, solid answer in the long response he finally gives. (HBP pg. 30-31)
Secondly, Severus acts very much as though he knows "about the plan" yet never manages to say a single piece of information about it before Narcissa or Bellatrix. Everything he knows about the plan is because they told him. At this point, it appears to me that he is gathering information for Dumbledore. It is never mentioned to Harry by Dumbledore, but that is small matter in this situation. Severus eventually would have figured that the actual target was Dumbledore and told him. Dumbledore would have kept it to himself so as not to worry Harry. Obviously, if Harry knew, he would not be able to restrain himself from attacking Draco or something. (HBP pg. 32-33)
Next point. Severus is extremely reluctant to give the Unbreakable Vow, and only does so in the end because Bellatrix refuses to believe he is on their side (and I agree with her).
[["If you are there to protect him...Severus, will you swear it? Will you make the Unbreakable Vow?” “The Unbreakable Vow?” Snape's expression was blank, unreadable. Bellatrix, however, let out a cackle of triumphant laughter. “Aren't you listening, Narcissa? Oh, he'll try, I'm sure...The usual empty words, the usual slithering out of action...oh, on the Dark Lord's orders, of course!” Snape did not look at Bellatrix. His black eyes were fixed upon Narcissa's tear-filled blue ones as she continued to clutch his hand. “Certainly, Narcissa, I shall make the Unbreakable Vow,” he said quietly.]] (HBP pg. 35-36)
As you see, it is Bellatrix's goading words that bring him to the conclusion that he can do nothing else. We know that Severus is very intelligent magically and is a very successful Occlumens, but he is a weak person. If one notices, Severus follows whoever he thinks can give him the most protection. At first, he is angry with James and the other Marauders and is interested in the Dark Arts. This makes it appear to him that Voldemort is the wizard to follow. However, with Dumbledore fighting against his current cause, Severus must already have had extreme notions as to power of Dumbledore (most students get the impression that their instructors are more than they are, but in Dumbledore's case, I think it's hard to break the bounds of his power) and so, Severus realised that Dumbledore would offer more protection to him. It must have been an utter relief to him when Voldemort told him to become a double agent. Or it may have been that Severus "felt he wasn't doing all he could" and thought if he could get into the confidence of Dumbledore, that he would be more useful, hence the assignment. From that point, I believe Severus' story goes much as he states on pages 26 and 27 of HBP.
The next point I have to make is that when asked if Severus would carry out Draco's deed if it seems he would fail, Severus paused and it seems to me that he was considering whether or not to make the vow. However, he obviously did, and I believe it is for the same reason he agreed to make the vow in the first place.
Now, to the part where I cried. The scene in which Severus kills Dumbledore. Many believe that in that scene, Dumbledore begs for his life. I, however, do not believe this. On page 297 of SS, Dumbledore says, “After all, to the well-organized mind, death is but the next great adventure.” If there is someone who can prove Dumbledore to have had an unorganized mind, I ask that they present the evidence to me (firstname.lastname@example.org). I do not find it possible. If Dumbledore was the one to have told this to Harry, he must believe it. It is not in Dumbledore’s character to state such things unless he believes them to be true. Note how he frequently provided disclaimers for Harry whenever the information he told him about Voldemort was speculation. Now, to carry on. Dumbledore, in my opinion, was not begging for his life. I find it rather possible that he was, in fact, asking Severus to fulfill any duties that Dumbledore had asked of him previously. I believe that, if my theory is true, that Severus told Dumbledore about the plan to kill him already. I also believe that Severus informed him about the Unbreakable Vow he made. Dumbledore, being wise, would have understood exactly why Severus had done what he did. Knowing all the possibilities, I believe Dumbledore told, maybe even commanded, Severus to follow through with his vow if he had to.
[[Snape gazed for a moment at Dumbledore, and there was revulsion and hatred etched in the harsh lines of his face. “Severus … please …”]] (HBP pg. 595)
Severus paused. He did not kill Dumbledore directly. We know Snape is a talented Occlumens and can perform Legilimency. We also know that Dumbledore is a practiced Legilimens. I find it not only possible, but likely that they were communicating in some way. I believe that Dumbledore was doing two things. He was reminding Severus of the promise he made to follow through with the Unbreakable Vow and/or confirming that Harry was up there with them, and Severus was to make sure he got away. Either one of these could be the reason the was a look of “revulsion and hatred” in Severus’ face.
Another possibility for the reason behind Severus killing Dumbledore lies in his being weak, as I described before. We know for a fact that if Severus were to break his vow, he would have died.
[[”What happens if you break it, then?” “You die,” said Ron simply.]] (HBP pg. 325-326)
The look of “revulsion and hatred” could have been for himself, not willing to sacrifice himself to save Dumbledore.
Now for my final point. When Harry is chasing Severus in chapter 28, Severus does not fight back in any way. He blocks curses, but that’s all. In fact, the way he talks to Harry makes it seem as though he is trying to teach him something. He is confirming the importance of nonverbal spells. I don’t believe that the reason he does not attack Harry is that Voldemort wants to finish him off. Severus would still be able to inflict some manner of pain. He even stopped a Death Eater from using the Cruciatus Curse on Harry. The only time Severus lashed out at Harry was when his pride was hurt by being called a coward. What I concentrate on is that Severus, no matter his dislike and hatred for Harry, seems to be teaching Harry in this final scene with him. Teaching him what not to do when he must fact Death Eaters and eventually the Dark Lord in the future. I think it must have been his last attempt at doing what Dumbledore wanted him to do. I believe he was trying to help Harry in one final way because he had had so much respect for Dumbledore.
In conclusion, because of all the aforementioned aspects of the story, I believe Severus Snape to be no traitor to the Order of the Phoenix, but a continuing member, put more in harms way than any other member.
And that, my friends, is why I believe [insert name here] is not a [insert word here] and, indeed, has not turned to the side of [insert other name here].