They had agreed for Snape to kill Dumbledore so that the true owner of the Elder Wand would have died but technically not be beaten. Apparently the Elder Wand's power would then disappear. Malfoy screwed up the plan when he disarmed Dumbledore.
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Worry is a misuse of imagination.
Fantasizing about Guru champion BlAcK TuRtLe, however, is a great use of it.