Muggles' Guide to Harry Potter/Magic/Avada Kedavra

Avada Kedavra
Muggles' Guide to Harry Potter - Magic
Type	Spell (Curse)
Features	Kills instantly
First Appearance	Harry Potter and the Philosopher's Stone (though not identified until Harry Potter and the Goblet of Fire)

Overview

Avada Kedavra, also known as the Killing Curse, kills a person instantaneously and without injury. There is no counter for it, and only one person, Harry Potter, has ever survived it with Sacrificial Protection that has been endowed upon Harry Potter by His Mothers Love for Him. Along with the Imperius Curse and the Cruciatus Curse, the Killing Curse is considered to be one of the most terrible curses in the wizarding world, called the Unforgivable Curses; the use of any of the three on another human being is punishable by a life sentence in Azkaban.

Extended Description

Beginner warning: Details follow which you may not wish to read at your current level.

The Avada Kedavra curse is recognizable by the flash of green light and the rushing noise emitted from the caster's wand. Along with the Cruciatus Curse and the Imperius Curse, Avada Kedavra is considered to be one of the most terrible curses in the magical world, called the Unforgivable Curses; the use of any of the three on another witch or wizard is punishable by a life sentence in Azkaban. Avada Kedavra is an Aramaic phrase that means "I will destroy as I speak." Whether the words were chosen to be similar to the Muggle mock incantation "abra cadabra" is uncertain, but seems likely. It is also known by most wizards as Lord Voldemort's signature curse.

The Killing Curse is used numerous times by "evil" wizards, and very occasionally by "good" wizards:

In Harry Potter and the Goblet of Fire, it is used, but not identified, by Voldemort against Frank Bryce. We are introduced to it in Defence Against the Dark Arts class by the false Alastor Moody, it is used by Wormtail, at Voldemort's instruction, to kill Cedric Diggory, and it is later used against Harry by the returned Voldemort, though in this instance it is blocked by the Priori Incantatem effect.
In Harry Potter and the Order of the Phoenix, it is used by Lord Voldemort against Albus Dumbledore, and blocked by Dumbledore's pet phoenix, Fawkes, in the climactic battle in the Ministry of Magic. While it is possible that Bellatrix Lestrange uses it against Sirius Black in that same battle, the specific curse Bellatrix uses is not named, and Harry's expectation that Sirius will appear on the other side of the arch indicates that it was some other, non-fatal curse that simply happened to push him through the Veil.
In Harry Potter and the Half-Blood Prince, it is used once by Bellatrix Lestrange to kill a fox, and once by Severus Snape against Albus Dumbledore. It's likely that this is also the curse that Thorfinn Rowle attempted to use against Remus Lupin; the curse missed Lupin, hitting fellow Death Eater Gibbon.
In Harry Potter and the Deathly Hallows, there is description of multiple "green flashes" during the climactic Battle of Hogwarts, which leads us to believe that it is used by numerous wizards on both sides of that battle, late in Harry's seventh year. It is also used numerous times by Voldemort as he pursues the Elder Wand, and in a fit of rage against his own followers when he hears of the theft of the Cup Horcrux from Bellatrix's vault. It is also used by Death Eaters during Harry's escape from Privet Drive, and by Voldemort against Harry in the Forbidden Forest. The spell also may have been used, though remaining unnamed, by Mrs. Weasley to kill Bellatrix Lestrange in a duel during the battle. It should be noted that it is highly unlikely that Molly would use an unforgivable curse, especially Avada Kedevra, as even a spell as simple as Expelliarmus could kill someone if it were used with enough force and directed toward the heart.
While it is not actually stated, the "green light" that Harry remembers in his earliest memories, and which is mentioned in the first three books, is almost certainly from Voldemort's failed attempt to use the Killing Curse against Harry. In Harry Potter and the Deathly Hallows, we see in Voldemort's memory that he used it against Harry's father, mother, and finally against Harry himself.
In Harry Potter and the Goblet of Fire, we are told that when Barty Crouch was the head of the Auror department, Aurors were authorized to use deadly force against Death Eaters. One must assume that this occasionally took the form of the Avada Kedavra curse.

Analysis

The idea of causing death by a word is, perhaps, one of the ideas that makes one wish that magic actually existed, as the world would be, one supposes, much more civil if retribution for evil acts could be so quickly, easily, and finally applied. Of course, even in the Wizarding world, it is not so simple. From the outset, we have been told that the spell-caster's mindset is crucial to the operation of any spell, most notably with the Riddikulus spell used against Boggarts, and the Patronus spell used against Dementors. Additionally, Bellatrix Lestrange taunted Harry for his inability to cast a good Cruciatus curse during the battle at the Ministry. We imagine that in order to cast a Killing Curse that actually works, your mind must be filled with some emotion antithetical to your target. Whether that would be hatred, as required for the Cruciatus curse, or perhaps rage, which Voldemort seems to be so very good at, we do not know. A fair amount of mental power may also be required; it is possible that weaker wizards can't actually cast this spell.

Of course, the need for a specific mind set to cast the spell is not enough to prevent its being cast; the mind set can be learned. Hence the need to have use of the curse result in life imprisonment. As presented, there do not seem to be any mitigations to this life sentence, there does not seem to be any possibility that this curse may have to be used, for example, by Aurors in the line of duty. This may seem a little harsh, given that there may be circumstances where the killing curse is your only option; but again, these are books written for children, and children need that reassurance that crimes will be punished.

Questions

Study questions are meant to be left for each student to answer; please don't answer them here.

How did Harry Potter survive the Killing Curse and what His Mother do?
Did Lord Voldemort survive the curse when it backfired on him, and what state of life did he come into if he did?

Greater Picture

Intermediate warning: Details follow which you may not wish to read at your current level.

It is perhaps important that Harry never uses this curse himself, despite using the other two Unforgivable Curses. Voldemort's demise is caused by his own use of the curse against Harry, and the rebound of that curse.