Authors note: This is why I believe Snape did what he did, at the end of HBP, told in to form of a letter from Snape to Harry.

Snape's Truth

Harry Potter-

Please believe me when I say this, I never wanted to harm the headmaster, but I had no choice. And believe this, I will forever hate myself for what I have done, but again I had no choice.

Now before I begin, I must first assure you that I am not a Death Eater, and I am not, nor will I ever be, in league with the Dark Lord.

Now if I may start at the beginning:

A mere two weeks after the end of your 5th year, I was visited at my home, the location I will not divulge, by Narcissa Malfoy and Bellatrix Lestrange. The former informed me, that the Dark Lord had given her son, Draco, an impossible task. What the task was I did not know, but I lied, saying that I know, in hopes that Narcissa would then tell. Unfortunately she did not, fore her sister did not trust my loyalties, and rightfully so. I was then force, by Lestrange's suggestion, to take the unbreakable vow, of which you already know the conditions. I vowed to watch over Draco, to protect him, and to complete that task given to him if he should fail. I will never forgive myself for taking that vow.

When I knew it was safe, I immediately informed the headmaster of this development. He, being the greatest wizard to ever live, already seemed to know of the task given to Draco. Unfortunately, he would not tell me either. So I was forced to find what the task that was essentially mine was. I am sure that you are of my attempts to find the answer form Draco all year long.

On the night that you almost killed Draco, the magic of the vow caused me save his meaningless life, he finally told me his task. Again I returned to the headmaster, to inform him that he was in danger, but of course he knew. He impressed upon me that importance of keeping everyone of the students safe, the importance of my mission.

By now, I know you are smirking at me, as you have done that six years I you have known me, but I must continue to the night when it happened:

Flitwick, informing me that the Death Eaters had entered the school and that the Dark Mark was hanging in the sky, awakened me in the dead of night. And just as I told Miss Granger, and Miss Lovegood, Flitwick, did indeed pass out. I quickly made for the tower on which you and the Headmaster were trapped; yes I knew you were there. I found Draco already there, with four other Death Eater, egging him one. But he could not go through with the task, just as the Headmaster foresaw. Again I felt the vow pull me, but I resisted, I could not harm the man who was more of a father and a friend to me then anyone else. Then I heard his voice in my head, the Headmaster was calling me, and so I went to him. The please you heard him utter was not what you thought it was.

You must understand, that the Headmaster knew that he was to die this passed year, either at the hands of Draco Malfoy, a Death Eater, or myself, the last I prayed it would not be. He also knew that Draco would be unable to kill him, and thus be killed himself, something that the Headmaster would not have. Even if Draco could do the deed, the Headmaster did not want a child to kill, he wanted to preserve what was left of his innocence. So the deed fell upon me, and I did what was asked of me, not because of the spell cast by the vow, for I would have died before I ever harmed the Headmaster, nor did I do it out hatred, nor loyalty to the Dark Lord. I did it out of compassion for the man that lay before me, out of trust and out of undying loyalty to Albus Dumbledore.

And remember this, Potter, the Headmaster is, and never was, a stupid man. He always knows what he was doing, and always knew what those around were doing. He always had a plan, for everyone. Never forget that.

In regards to the Half-Blood Prince, I knew all along you had my book, any I wanted it that way. I was my last attempt to help you in the only way I could, in disguise. The book was to give you strength, protection, and knowledge.

If in our travels our paths meet, and you wish to kill me, please don't hesitate fore I deserve worse.

I am, always was, and will forever, like you, be Dumbledore's man through and through

-Severus Snape

This letter has been enchanted to only appear for Harry Potter's eyes only, and will destroy itself upon his completion of it.