Lord Voldemort
Voldemort's forces attack Hogwarts because Harry is searching for a Horcrux inside Hogwarts castle. During the battle, Voldemort kills Snape because he believes that Snape is the current Master of the Elder Wand (because Snape killed Dumbledore), but Voldemort doesn't know that the wand actually belongs to Draco, who was the one to disarm Dumbledore.