How dumbledore died?

Professor Snape uses the Avada Kedavra curse against him at the end of The Half-Blood Prince. This was a pre-arranged deal between Snape and Dumbledore, as we learn in Harry Potter and the Deathly Hallows-Dumbledore preferred that Snape kill him since he knew death was coming and he wanted it to be painless. He did not want to have to deal with getting attacked by Fenrir Greyback or getting tortured by Bellatrix Lestrange.