Partly because he's a reasonable person, and doesn't aim to kill. Also, he may have not been physically able to do so. [ professor moody said that the most they could do to him was give him a nosebleed] and, voldemort has the horcruxes, so he wouldn't have been able even if he could produce an avada kedavra curse.
There is likely a good reason that the Sectumsempra is not an unforgivable curse. It's mostly because Sectumsempra is Snape's personal curse that he created. It is not a widely known or used spell, and it is one that he kept secret — which means it can't be added to the list since no one really knows about it.
It's possible someone tried to take out Voldemort with a killing curse, but failed due to who they were taking on. Furthermore, Voldemort had eight active horocruxes for most of the time he was in power. Anyone fighting in the Order of the Phoenix would've known this and realized trying to kill him was futile.
It is also said to be a fairly complicated spell. It depends on a persons willingness to kill someone and also the power behind the wand. To quote Barty Crouch/Mad-Eye Moody in 'Harry Potter and the Goblet of Fire', “Avada Kadavra is a curse that needs a powerful bit of magic behind it.
Why Harry DIDN'T Use Avada Kedavra on Voldemort - Harry Potter Explained
Did Molly cast Avada Kedavra on Bellatrix?
The author explicitly states that both women were fighting to kill, despite Molly refraining from casting the Unforgivable curse Avada Kedavra. The result of the duel actually comes about as a result of nothing more than spectacular accuracy on Weasley's part.
What happens if you use Avada Kedavra on a dementor?
From the Dementor's webpage: "They seemingly could not be destroyed, though their numbers could be limited if the conditions in which they multiply were prevented from forming, implying that they did die off eventually."
The intent of the caster is a vital component of the spell. While Snape was still able to produce Avada Kedavra without any intent to kill, being a great wizard, the mix of hesitation and necessity probably resulted in the spell being blue.
Although it is not confirmed to have been the Killing Curse, it is highly likely that Ron attempted to kill Voldemort's snake, and last Horcrux, Nagini, with Avada Kedavra. This is because, in the movies, Ron can be seen firing out a green spell at the snake, and it looks just like the Killing Curse.
The Disarming Charm became Harry Potter's signature spell, and it notably caused the final defeat of Lord Voldemort during the Battle of Hogwarts by rebounding his Killing Curse, because the Elder Wand recognised Harry as its master through the spell connection, and thus strengthened the Disarming Charm to the point ...
Effect. Unforgivable Curses are the three most powerful and sinister spells known to the wizarding world, and are tools of the Dark Arts. They were first classified as "Unforgivable" in 1717. They are the Killing Curse, Avada Kedavra, the Cruciatus Curse, Crucio, and the Imperius Curse, Imperio.
Vulnera Sanentur was the song-like incantation of a healing spell which was used to heal deep wounds, which could also be used as the counter-curse for the curse Sectumsempra.
After the Wizards' Council was reformed into the Ministry of Magic, tighter restrictions were placed on the use of certain kinds of magic. The Imperius Curse was deemed by the Ministry to be Dark magic, and, along with the Cruciatus and Killing Curses, were declared "unforgivable" in 1717.
The Killing Curse was created in the early Middle Ages by Dark witches and wizards. Primarily, the curse was used to quickly slay opponents during a duel. In 1707, the Wizards' Council was restructured into the Ministry of Magic, which allowed for more strict restrictions on certain types of magic.
Harry used the Imperius Curse multiple times during the Gringotts heist to accomplish their mission. Harry never used the Killing Curse, Avada Kedavra, due to his moral stance against murder and his personal connection to its victims.
It did backfire on Voldemort but just not the degree it did the first time. It failed to destroy his body because it killed the piece of hit soul that was in Harry forming the horcrux but it did knock him off his feet and actually rendered him senseless.
In the film, Bellatrix casts a blue spell at Ginny Weasley instead of a Killing Curse, who deflects it using a Shield Charm. In the book she just avoids it.
The Killing Curse, as an "unblockable" Curse, cannot be intercepted by another spell, except in circumstances of Priori Incantatem, where the caster and his opponent's wands and spells are locked together. An explosion or green fire may result if the spell hits something other than a living target.
In the Triwizard Tournament, he used Expelliarmus against Voldemort's Avada Kedavra, leading to the rare Priori Incantatem effect. The charm's non-lethal nature is also why Harry used it against Draco Malfoy in the Room of Requirement, despite the unintended consequence that followed.
The thing with Avada Kedavra is that it is essentially unblockable (ie. Protego, and other shield spells are ineffective against it.) Only dodging it, an effect like Priori Incantate, or putting something solid between you and the caster (for example: a Phoenix, a statue, or another living creature) will stop it.
The first time you can learn Avada Kedavra is at the end of the "In the Shadow of the Relic" quest. If you said no the first time, you'll have the opportunity to learn the killing curse from Sebastian later, depending on whether you snitched on him.
Do you get sent to Azkaban if you use Avada Kedavra?
Fans have always been eager to know more about this killing curse; some might have also wanted to use it. So they got their wish come true, as they can use Avada Kedavra in Hogwarts Legacy. But in the Wizarding World, one would be imprisoned in Azkaban for using the killing curse.
