Voldemort was so shocked that his main DE had died, and he'd been the killer, that he'd almost forgotton about Harry. The spell came at him, and he blocked it at last minute, but as he did so, the red sparks turned intno flames and still roared at him. He used a protective charm, a water spell, but the flames were determined. He cried out in shock, "YOU!" were his last words, his finger poiting at Harry, before the flames hit, and he knew his time had come.