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At one point in the last book it said that Dumbledor was overheard arguing with Snape. He was telling Snape to do something and Snape didn't want to do it, but finally relented. It was similar to the conversation that Harry and Dumbledor had before they left Hogwarts for the cave. Dumbledor told Harry that he had to promise to save himself and leave even if it meant that Dumbledor would die. Harry didn't want to agree with it, but he eventually promised that he would. I think that Snape promised Dumbledor that if he had no choice, he would kill Dumbledor for the greater good. Meaning, it was more important for Snape to remain close and trusted by Voldemort in order to allow Harry access and assistance when the time came.
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exactly thats what my theory is anyway. I think that Dumbledore did it just so that Snape could stay in the good graces with Voldemort. and that Snape will help Harry out in the end.