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Intro. Malfoy could not kill Dumbledore, so according to Dumbledore's plan, Snape came out for this, since Snape works for Dumbledore. He pretends that he works for Voldemort and in order for Voldemort to believe Snape, Snape must kill Dumbledore in front of Voldemort's henchmen

Albus Dumbledore

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