Oh yeah!! Its funny how we miss the finer details when we are in the thick-of-the-moment!!! Of course, the non-verbal spells!!! But the question is can it be used along with the verbal spells?? If so, which one gains importance?
You know what i think, Snape must have used a non-verbal curse that makes you repeat your last words over and over and then throws you off your feet....rmbr DD going "Severus, severus...." before being thrown off the tower. And once you have used a nonverbal spell whatever comes out of your mouth has no effect on the victim!!! Sounds cool huh
But one thing could be this, even if Avada Kedavra didn't kill DD, then the looooong fall from the tower to the great hall certainly would have...he's not exactly a spring chicken, young and sprightly!! To that one might ask, why not find a way to protect himself....he was the greatest wizard after all. Many answers to that one....
1. He did and this is all a trick to stump us (JKR didn't cry when she killed DD off, but she claimed to have shed a few tears when she killed Sirius)
2. He was tooooo weak to think straight
3. He didn't want to take the chance of voldi ever finding out he was alive, so he died
4. He wanted harry to be driven to stop the dark lord for good as Harry was showing a remarkable aversion to the prospect of having to commit murder!!! So he sacrificed himself. (Hey, its possible you know