Screaming from the agony
Most people seemed to believe the killing curse to be painless.
It was not.
Avada Kedrava didn't simply sever the connection between soul and body as some wixen were prone to think; no, what the spell actually did was shatter the soul. When cast, the killing curse would shatter the soul of the target, the fragments so, so small that they were easily consumed by the natural magic of the world.
So, when Voldemort uttered the words 'Avada Kedrava' at his prophesied enemy, the expected thing happened.
But the unexpected also happened.
Harry Potter died. That much was obvious, the light left the boy's eyes and his skin paled; but then, suddenly an oppressive and desperate magic reached out to Voldemort. He quickly realized that said magic belonged to his enemy, but before he could react, the boy's magic reached out to his very soul.
Potter's magic was strong and desperate enough to simply tear off a piece of Voldemort's own very soul.
For a fraction of a second, it seemed nothing had happened. But then the pain hit him and Voldemort screamed from the agony. The last thing he saw was both his own green spell coming back at him and one very empty eye staring at Voldemort, devoid of life itself.