Plot hole: One of Snape's memories show Lily telling baby Harry to be safe, be strong. That had to have occurred before Voldemort killed Lily. Snape was not in the house until after Lily was killed and Voldemort was gone. Only Lily and Harry should have had that memory, but not Snape.
Plot hole: Immediately after the confrontation between Harry/McGonagall and Snape in the Great Hall, Harry runs up to Ravenclaw tower against crowds of students running the opposite direction. However, all the students would have been in the Great Hall during that confrontation, and therefore would have been coming from the same direction as Harry.
Answer: It's a little complicated. Voldemort actually died by his own hand, though unintentionally. Voldemort believed he commanded the Elder Wand and cast the Avada Kedavra curse at Harry with it. However, Harry was actually the Elder Wand's master. Because wands are somewhat sentient, the Elder Wand recognized Harry as his master, so the killing curse rebounded off him, and went back to Voldemort, striking him dead. All of Voldemort's Horcruxes had been destroyed, so he was no longer protected by them. At the same time, Harry cast the Expelliarmus charm, causing the Elder Wand to be ejected from Voldemort's hand into his.
raywest ★