In June 1997, Harry and Dumbledore went to search for Horcruxes. They found the fake Horcrux - Regulus's locket in the cave. In order to obtain the Horcrux, Dumbledore drank the potion and was dying. Harry returned to school with Dumbledore and the fake Horcrux. That night, the Death Eaters entered Hogwarts with the help of Malford. Malfoy disarmed Dumbledore and Snape killed Dumbledore.
If the development of history had changed, Severus did not kill Dumbledore
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