There are many clues in the HARRY POTTER series which tells us that Dumbledore is not actually dead, But has pretended himself to be dead. Which might be a part of his Further PLANNINGS(with the order of phoenix including SNAPE!!)
This theory tells us that Snape is LOYAL to dumbledore. Some of the clues are:
1. The Trust On Severus
Dumbledore shows extremely, extremely high trust on SEVERUS SNAPE in the whole 6th part.
At last he said, 'I am sure. I trust Severus Snape completely.' harry breathed..... (513. HBP)
Merely after 2-3 paragraphs from there dumbledore gets Irritated from Harry (About what he was saying Against Snape) and tells him not to speak anything against Snape. Although harry gave quite strong reasons against Snape for Dumbledote to take action or investigate against him, but he ignored all of what harry told him, as if he was telling a Joke or something sort of that!!!!!
'Snape's what's happened! He told Voldemort about Prophecy, it was him, he listened outside the door, Trelawny told me!" ........ ........ 'He hated my dad like he hated sirius! Haven't you noticed, Professor, how the people Snape hates Tend to end up dead?' ....... ....... But he's a Very good Occlumens, isn't he, sir? said Harry, whose voice was shaking with the effort of keeping it steady. 'And isn't Voldemort convinsed that Snape's on his side, even now? Professor... How can you be sure Snape's on our side?'
Dumbledore did not speak for a moment; he looked as though he was trying to make up his mind about something. At last he said, 'I am sure. I trust Severus Snape completly.' ...... ...... 'Well I don't!' he said, as loudly as before. 'He's up to something with Draco Malfoy right now, right under your nose, and you still-' ..... ..... 'Enough,' said Dumbledore. He said it quite calmly, and yet Harry fell silent at once;
As we have seen, Dumbledore was The first among the very few people who actually believed Harry that Lord Voldemort Was back, and so it looks quite awkward that he dosen't even tries to think what Harry had said about.
Why Dumbledore Doesn't listen to harry seriously? There can be one and only one reason that dumbledore is aware of all these things or, more clearly snape has informed him everything and there is a secret plan between them.