Other mistake: In both the books and the movies, Harry's Invisibility Cloak can always be seen until it is worn. In Diagon Alley, just before the Gringott's break-in, Ron pulls out the cloak to cover Harry and Griphook, but it turns invisible before he puts it on them. Ron merely mimes the motion of covering them.
Other mistake: In Snape's memories, Trelawny's prophecy is the wrong one. If you listen closely you can hear her saying "master shall be reunited once more". That's the prophecy that she told Harry in PoA, not the first prophecy Snape half-overheard Trelawny telling Dumbledore.
Other mistake: In the final scene when Harry and Voldemort "connect" their wands, there is a close-up of Harry's face. Although the only light source seems to be the ball of fire between him and Voldemort, Harry's face is lit from his left.
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 ★