I think Dumbldore DID tell Snape to kill him, but the reason is a bit obscure currently. Despite all of Snapes justifications to Bellatrix, the one thing he didn't explain, and Bellatrix didn't know about was why he returned to Grimmauld place and alerted the Order to the fact that Harry and Company had gone to the ministry. I don't think Snape is evil. But I do think Dumbledore is dead, though we may get to enjoy his company in the office portrait if Harry ever has an opportunity to be there.