I am researching the capabilities and limitations of the Killing Curse in Harry Potter (Avada Kedavra). Can the Killing Curse be used to kill more than one? Or, is it only able to kill one foe?
The Killing Curse can only be used to kill a single individual. In plain words, if you want to "neutralize" two opponents, you need to cast it twice.
It is my understanding Avada Kedavra means immediate death, even if it hits only an arm or a leg. Think of it as being literally touched by death, and it is a very powerful spell.
"Avada Kedavra's a curse that needs a powerful bit of magic behind it — you ... them at me and say the words, and I doubt I'd get so much as a nosebleed."
— Barty Crouch Jr, disguised as Alastor Moody*.
Considering the above mentioned, I can picture three scenarios in which Avada Kedavra could kill more than one person at a time:
- Two people who are holding hands are hit at the same exact spot by a single AK cast by a particularly powerful spell.
- Siamese twins still attached to one another.
- A pregnant woman killed along with her yet unborn child(ren).
My guess is the wizarding world doesn't exactly long for the day Wizardkind applies Muggle warfare principles to magic. This would be truly devastating. Voldemort and his followers would look like a bunch of kids throwing a tamper tantrum.
I interpret canon as suggesting it's possible to kill more than one person with a curse; Peter Pettigrew killed thirteen Muggles with a single curse, an act which was subsequently blamed on Sirius Black. When Harry catches the Knight Bus in Prisoner of Azkaban, the conductor, Stan Shunpike, reads Harry the news about Sirius Black from the Daily Prophet:
While Muggles have been told that Black is carrying a gun (a kind of metal wand which Muggles use to kill each other), the magical community lives in fear of a massacre like that of twelve years ago, when Black murdered thirteen people with a single curse.
Prisoner of Azkaban - page 34 - Bloomsbury - chapter three, The Knight Bus
What it doesn't say -- anywhere in Prisoner of Azkaban, as far as I could find -- is exactly what curse Peter used. It's definitely possible that Peter used Avada Kedavra, because that is the classic killing curse. DVK pointed out to me, though, that perhaps Peter's goal all along was to cause a lethal gas line explosion that would totally wreak havoc and allow Peter to escape. In this scenario, a curse like Incendio (Incendio sets fire to something) or Expulso (Expulso is specifically a curse that blows up a target.).
I think the following passage suggests that it is possible for Avada Kedavra to kill more than one person at a time. In Order of the Phoenix, Voldemort possesses Harry in the hope that Dumbledore will kill Harry as a means to kill Voldemort:
‘Voldemort’s aim in possessing you, as he demonstrated tonight, would not have been my destruction. It would have been yours. He hoped, when he possessed you briefly a short while ago, that I would sacrifice you in the hope of killing him.’
Order of the Phoenix - page 730 - Bloomsbury - chapter thirty-seven, The Lost Prophecy - Albus Dumbledore
At this point, though, the full extent of Voldemort's Horcruxes were not known. With the Horcruxes in place, no, Voldemort would not have died. But if he had no Horcruxes, the passage suggests that both Harry and Voldemort might have been killed by Dumbledore (That is, had Dumbledore chosen to cast Avada Kedavra; Dumbledore believed there were things far worse than death.).
Also, of note, if a witch or wizard mispronounces a spell while casting it, the effects of the spell can change. In Order of the Phoenix, during a DA meeting, Cho Chang mispronounces Expelliarmus and sets Marietta Edgecombe's arm on fire. Could a mispronunciation of Avada Kedavra cause unusual results? Perhaps.
As you can read in the Harry Potter Wikia:
Creation: The Killing Curse was invented during the early Middle Ages, by Dark witches or wizards. The curse was created primarily as a means of quickly and efficiently slaying one's opponent in a duel.
I interpret this to mean that the Killing Curse (also known as the Avada Kedavra Curse) can be used to kill just one being.
As far as I know, there are no direct multi-target spells, but you can cast a spell more than once. Like this, you can target more than one being. I wrote "direct" because you can target a spell to flood a room, so every being inside will be affected.
Also the spells used to damage other beings were developed to duel them -- or to show who is more powerful -- so there is normally no use for these spells to be used against more than one being.