There's a huge discussion about the why Snape killed Dumbledore all over the web. Some believe that the chapter "The Unbreakable Vows", plural Vows, indciates that Dumbledore made Snape make an Unbreakable Vow, concerning Snape helping Harry complete the destruction of the Horcruxes. Why would Dumbledore force Snape to make the Vow if he was able to help? Maybe Dumbledore knew or even planned his own death for the sake of destroying Voldemort.

Also, I want to add that Rowling established that there is no return from the killing curse in one of the previous books. I think she said this to avert speculation about the return of anyone whom she decided would need to be dead. I don't think Dumbledore is going to be coming back.
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