An original editorial by Logan Anbinder
Note: According to the prophecy, Harry has the power to vanquish Voldemort. Given Dumbledores declaration that [Voldemorts] failure to understand that there are things much worse than death has always been [his] greatest weakness (OotP pg. 814, US Hardback); I dont think Harrys necessarily going to defeat Voldemort by killing him. However, its definitely a possibility because destroying all the Horcruxes is a prerequisite for killing Voldemort, as opposed to another method of defeating him that might not require the destruction of the Horcruxes. The following editorial assumes that Harry will vanquish Voldemort by killing him.
When Bellatrixs spell knocks Sirius through the veil at the Ministry, Harry is (quite understandably) enraged. He catches up with her in the Atrium after her flight from the Department of Mysteries, and she taunts him. Then "hatred rose in Harry such as he had never known before" (OotP pg. 810, US Hardback), and he attempts to cast the Cruciatus Curse on her:
Bellatrix screamed. The spell had knocked her off her feet, but she did not writhe and shriek with pain as Neville had
"Never used an Unforgivable Curse before, have you, boy?" she yelled
"You need to mean them, Potter! You need to really want to cause pain - to enjoy it - righteous anger wont hurt me for long
(OotP pg. 810, US Hardback)
We learned in Goblet of Fire that Avada Kedavra (and presumably the other two Unforgivable Curses) are "curse[s] that need a powerful bit of magic behind [them]" (GoF pg. 217, US Paperback). From this passage in Order of the Phoenix, we learn that the caster must be not only powerful, but guided by a certain mentality - namely, the willingness to cause suffering for sufferings sake, rather than as revenge for a loved ones death. Though Harry is angered, it is "righteous anger," not the type of cruel rage best suited for casting Unforgivable Curses. As a result of his justified anger, he is unable to cast a successful Cruciatus Curse; instead, he causes only momentary pain.
Harrys anger toward Voldemort, for killing his parents, is the same type of "righteous anger" he feels toward Bellatrix for killing Sirius, and is obviously stronger. This begs the question: If Harry is unable to cast a fully powerful Cruciatus Curse on Bellatrix with the type of hatred he has for her, what hope does he have of casting an Avada Kedavra on Voldemort?
The answer may lie in the one of the three Unforgivable Curses not yet touched upon in this editorial. Impostor Moody demonstrates the effects of the Imperius Curse on Harrys classmates in Goblet of Fire:
Harry watched as, one by one, his classmates did the most extraordinary things under its influence...Neville performed a series of quite astonishing gymnastics he would certainly not have been capable of in his normal state.
(GoF pg. 231)
If the Imperius Curse can make people capable of things they can not normally accomplish, why couldnt this be used as a method to allow Harry to successfully cast Avada Kedavra? If Harry were to be Imperiused, with his consent, he would be able to use the Unforgivable Curses, regardless of his previously encountered problems casting them. (I dont think that Harrys ability to throw off the Imperius Curse is an issue here. The resistance is probably not something that happens automatically. Harry could make a conscious decision to accept the casters orders instead of fighting against them.)
A few examples of the Imperius Curse's skill-altering capabilities have already been mentioned in the series; the first, when Viktor Krum was placed under Impostor Moodys control in the maze, during the third task of the Triwizard Tournament:
Harry heard Krums voice. "Crucio!"
(GoF pg. 626-627)
...looking to his right, [Harry] saw Cedric jerking and twitching on the ground, Krum standing over him.
While we know students at Durmstrang are taught the Dark Arts, it has not been specifically stated whether or not Krum had learned the Cruciatus Curse beforehand, or was only capable of using it under the Imperius Curse. However, another, more blatant example of such coercion is mentioned in passing in HBP, when Hermione is reading the Daily Prophet:
"Oh, and how horrible, a nine-year-old boy has been arrested for trying to kill his grandparents, they think he was under the Imperius Curse..."
(HBP pg. 457, US Hardback)
Assuming the boy was using Avada Kedavra, this passage is a confirmation of the Imperius Curses ability to enable the victim to use Unforgivable Curses, since such a young child would obviously be unable to cast the spell himself. (Admittedly, the passage says "trying -" this could mean "trying to use Avada Kedavra but was unable to." However, it could also mean "trying to use Avada Kedavra but was stopped in time.")
Assuming this theory is correct, and Harry must be under the Imperius Curse in order to kill Voldemort, then we must consider the possible casters of the curse on Harry. As the Imperius Curse is also one of the Unforgivables, the same rule applies to it as it does the other two: the caster needs a certain sinister (or at least grim) mentality to use the spell. This pretty much takes any of Harrys classmates off the list. As there are several Aurors in the Order, it is possible that one of them could Imperius Harry. Since they have presumably used Unforgivable Curses in the past on Dark wizards in order to capture or defeat them, we can deduce that they are fully capable of doing so. (While we havent seen any Auror actually use an Unforgivable, Impostor Moody did, which indicates that Moody is also able to use them. Crouch Jr. wouldnt have wanted to draw attention to the fact that Moody was casting the curses, unless it was a feat the regular Moody could accomplish). Lupin may also be a candidate since he and Sirius were poised to kill Pettigrew for his treachery, in the Shrieking Shack, in Prisoner of Azkaban. They were almost certainly going to use Avada Kedavra, as Sirius and Lupin stood with "wands raised" (PoA pg. 375, US Paperback). However, there is another member of the Order whose casting of the Imperius Curse on Harry could potentially make for a much more exciting plot; one we have actually seen cast an Unforgivable Curse before: Snape.
With JKRs declaration at the NYC reading that Dumbledore is definitely dead, its now safe to assume that the spell Snape cast on top of the tower was a real Avada Kedavra. This would mean he is able to cast the other two Unforgivable Curses as well, including the Imperius Curse. Plot-wise, imagine a scenario in which Harry, still mistrusting of Snape despite his explanation of the events on the tower, must put himself under Snape's complete control to defeat Voldemort, giving both Harry and Snape a chance to strike back at the man they despise. (Obviously, this part of the theory only works if Snape is good, which I believe he is. However, thats a matter for another editorial; or rather, the ten thousand other editorials that have already been written about that very topic).
Of course, its possible that Harrys going to defeat Voldemort by merely destroying all the Horcruxes, then placing a rock in front of the veil so that Voldemort trips and falls in. But if he does have to use Avada Kedavra, he now has a way to do so.
Posted by: Amy