Harry Potter killed Voldemort with the Disarming Charm, also known as Expelliarmus. This spell was Harrys signature dueling spell, and he used it to disarm Voldemort of the Elder Wand, which was the source of his power. Although Voldemort attempted to kill Harry with the Killing Curse, Harrys Expelliarmus spell caused the curse to rebound and kill Voldemort instead. Therefore, it was not Harrys spell that killed Voldemort, but rather the curse that Voldemort cast himself.