Okay, so what did go down between Draco, Snape, Dumbledore, and two random magical cabinets? As was previously mentioned, Draco unlocked the pathway between the two Vanishing Cabinets once Montague was freed from them, using them as a passageway to help Voldemort’s bands of Death Eaters get into the school — including Bellatrix Lestrange (Helena Bonham Carter) and Fenrir Greyback (the late Dave Legeno), two of the Dark Lord’s most violent and dangerous henchmen. Dumbledore was crafty and strategic, but he never would have wanted such horrifying wizards near his students; all he wanted to do was save Draco from committing murder with the help of Severus Snape.
See, Snape and Dumbledore struck up a deal long before this fateful evening. Dumbledore, thanks to a Horcrux hunt gone wrong, was already struck with a terminal illness, and he and Snape knew Draco had been ordered to kill Dumbledore at Voldemort’s personal request. As a result, Snape agrees to stage Dumbledore’s murder, as his soul will remain intact knowing that he helped an old, dying man pass on peacefully. This isn’t apparent to Harry or anybody else at the moment of Dumbledore’s death; he later discovers it thanks to Snape’s final memories as he, too, lays dying. Basically, Fred and George Weasley accidentally gave Draco a clever passageway… but they definitely didn’t kill Dumbledore.