The Deathly Hallows included the Elder Wand, which is what Voldemort was using to fire his killing curse, the spells collided and because Harry was the master of the wand, the killing curse was forced back onto Voldemort, killing him.

Why does Harry never use Avada Kedavra?

Why Harry Didn't Cast Avada Kedavra

Avada Kedavra was Lord Voldemort's signature spell. ... Voldemort simply didn't show any remorse for those he killed, so regret was never an issue when another opportunity to murder arose. By the time he faced Harry in their final duel, both wizards could have used the Killing Curse.

Why did Voldemort die like that?

But Voldemort's killing curse rebounds, reflected by Harry's blood protection and the Elder Wand's allegiance to him, while Harry's Disarming charm wrenches the Elder Wand from Voldemort's hand to his. With all his Horcruxes destroyed, Voldemort dies instantly.