While Dumbledore is away, a gang of Death Eaters, led by Draco Malfoy, sneaks into Hogwarts through the vanishing cabinet in the Room of Requirement. They battle the Order of the Phoenix, and when Dumbledore and Harry return, Dumbledore freezes Harry as Draco arrives to Disarm Dumbledore (HBP27).
After a standoff, Snape arrives and kills Dumbledore, and the Death Eaters flee the school. Harry then discovers that the locket is not the real Horcrux, but rather that it was replaced by a fake one from R.A.B. (HBP28).
McGonagall, now acting as headmaster, meets with the Order and with the heads of houses, determining that students will be allowed to stay for Dumbledore’s funeral (HBP29).
Snape Assists Dumbledore?
Discussants on Episode 3 of the Mugglenet Academia podcast mentioned the resonance of Snape's murder of Dumbledore with current debates regarding assisted suicide. --Hufflepuffskein