With which spell did Harry Potter kill Voldemort?

Question

As I've seen in the movie Harry Potter and The Deathly Hallows (part - 2), Voldemort's last spell produced a green light. I think it was Avada Kedavra. On the other hand, Harry's spell which was locked with it was reddish. Harry's spell destroyed Voldemort's body slowly after Voldemort cried for a while.

From what is shown in the movie franchise and documented on the Wikia,

• Avada Kedavra produces a green, not reddish, light.
• Avada Kedavra doesn't destroy the body of the victim.
• Death by Avada Kedavra is instant and painless.

This suggests that Harry's spell which killed Voldemort wasn't Avada Kedavra. Then, what spell was that?

Answer 1

Harry used his signature dueling spell: Expelliarmus. Due to a combination of the Elder Wand's true owner being Harry, not Voldemort, and the spells colliding, Voldemort's Avada Kedavra rebounded upon him (again). Since at that point all of his horcruxes had been destroyed there was nothing to keep him alive.

Harry heard the high voice shriek as he too yelled his best hope to the heavens, pointing Draco’s wand:
“Avada Kedavra!”
“Expelliarmus!”

Then:

Harry saw Voldemort’s green jet meet his own spell, saw the Elder Wand fly high ... spinning through the air toward the master it would not kill, who had come to take full possession of it at last.

And:

Tom Riddle hit the floor with a mundane finality, his body feeble and shrunken, the white hands empty, the snakelike face vacant and unknowing. Voldemort was dead, killed by his own rebounding curse, and Harry stood with two wands in his hand, staring down at his enemy’s shell.
_{Chapter Thirty-Six, The Flaw in the Plan - Harry Potter and the Deathly Hallows}

I haven't seen the last two movies but it seems as though the final scenes differ from the books pretty substantially. As the quotes above state, Voldemort's death is instant (as you'd expect from Avada Kedavra) and his body remains intact.

Answer 2

Okay, you asked about the movie and tagged this post "movie" so that is what standpoint I'm going to answer from.

The final casting of spells starts at around 2:26. Neither Voldemort nor Harry incant any spell at all. They just aim and cast their spells nonverbally. So technically the answer is the spell Harry used to defeat and kill Voldemort in the movie is unknown. As I mentioned in my comment, and Anthony Grist points out in his answer, in the book it is Expelliarmus.

I did note that when Voldemort lost his wand in the movie scene, the Elder Wand sails through the air from Voldemort to Harry. This might be indicative of Expelliarmus, although if you compare that to other uses of Expelliarmus in the movies, Expelliarmus usually just flings a wizard's wand away in a random direction. That said, in the movie Deathly Hallows I, Hermione disarms Mundungus Fletcher with Expelliarmus and Dung's wand does sail through the air to Hermione and she catches it. But getting back to Harry and Voldemort, technically no spell is said outloud by either.

Answer 3

In the movie, and considering only canon from the movie series, it looks very much like Voldemort died because his last horcrux (Nagini) was killed. I'm going to get a lot of down votes for this, but there's literally no reason to think that Harry killed Voldemort in the movie.

1. Based on the visible evidence, Harry cast Expelliarmus. His spell was green, and when it reached Voldemort it did actually take away his wand. The Disarming Charm isn't a killing spell.

2. Based on the clip, after the green spell reaches Voldemort and expels the Elder wand, it takes another 14 seconds before Voldemort starts to disintegrate. If Harry's spell killed Voldemort, why did it take so long? Is it some special new slow-killing spell that he developed just for this battle? Avada Kedavra kills instantly evidently in the movies.

3. The idea of Avada Kedavra rebounding is from the books, and is never seen or mentioned in the films. Even if it did rebound, why would it take so long to take effect?

4. Harry Potter has never killed anyone. (technically) This is a hero trope, and especially a movie trope, that the hero disarms/disables/etc the villain because of their personal code of Thou Shalt Not Kill (like Batman..). I don't know why Hollywood thinks this way, but it seems to be pretty common that good-guy heroes will defeat villains without actively killing them. Maybe the villain kills themself. Maybe some other character kills them. Whatever, it seems to be attractive to leave the hero without a stain on their conscience.

Answer 4

The only spell I know that could disintegrate someone would be Deletrius. Maybe he cast Deletrius while the movie followed the wand. I don't know!