The Death Eaters and the Dark Lord don't simply cast Avada Kedavra on everyone all the time because it doesn't suit their long-term goal of ruling the world and having a wizard-run society.
Hogwarts Legacy's Ranrok Dragon is Technically Not 'Alive'
There will likely be fans upset that the final boss cannot be dealt with instantly with Avada Kedavra, but because the curse only canonically works on living beings, Hogwarts Legacy may have found some clever wiggle room with Ranrok Dragon.
Why didn't Harry use it on the locket, or cup…? A Horcrux is considered destroyed when it's damaged beyond repair. In the case of living Horcruxes, “damaged beyond repair” would mean dead. So, yes, Avada Kedavra can destroy a Horcrux, as long as that Horcrux is a living being.
In the books Bellatrix is clearly playing with her food. And in her overconfidence Bellatrix fails to deflect, dodge or block an immediately lethal spell from Molly Weasley. If you'd like to explain film Bellatrix's actions overconfidence prevented her from using Abada Kedavra straight away.
Though Harry encountered many opportunities that deemed Avada Kedavra necessary, it remains as one of the Unforgivable Curses Harry Potter never cast. For one, he viewed the spell as an immoral practice commonly used by users of the Dark Arts.
In order to conjure the avada kedavra curse, you have to want to kill your victim. We all know that Voldemort could easily kill a child without an ounce of remorse... but not Snape. Snape didn't want to kill Dumbledore, and this was why the spell was blue instead of the usual green.
None of the good guys dare use the Killing Curse throughout the events of the movies and books, no matter how high the stakes and how difficult things are. But Ron Weasley is the exception, giving it a go in the Deathly Hallows: Part 2 movie when himself and Hermione look destined to die via Nagini.
Sirius Black couldn't have countered Bellatrix's killing curse becaue the killing curse isn't something that can traditionally be countered with the disarming charm.
Cruciatus Curse ('Crucio')
Harry tried to use this spell himself. The first time was against Bellatrix after she had killed Sirius. His attempt was unsuccessful because, as Bellatrix mockingly explained: 'You need to really want to cause pain – to enjoy it – righteous anger won't hurt me for long…'
So, Voldemort had to kill Snape for the wand's allegiance to switch. Voldemort knew he couldn't cast Avada Kedavra with the Elder Wand to kill its master, so he ordered Nagini to be the one who killed Snape. In reality, the Elder Wand belonged to Draco, as he was the one to disarm Dumbledore in the Astronomy Tower.
Yes, if the curse is powerful enough. Hagrid is half-giant, and it is said that giant-blood can “absorb” some of the power in spells. ... For instance, Voldemort's Avada Kedavra can probably kill Hagrid with ease. Yes, but it would require a stronger than average wizard casting it to have a killing effect.
Once you return to the castle, you can visit Sebastian in the Undercroft in the Defence Against the Dark Arts tower, and ask to learn about it. This is the same case with the rest of the Unforgivable Curses. If you decide to turn Sebastian in before you learn the curses, you will lose your ability to learn them.
Phoenixes were semi-protected from the Killing Curse, due to them being immortal. In 1996, Fawkes swallowed one intended for Albus Dumbledore, causing him to burst into flame and die instantly. However, he then was reborn from his ashes. This makes phoenixes resistant to the Killing Curse.
In Deathly Hallows, Hermione faced a real test of her physical and psychological stamina. After being on the run from a bunch of Snatchers, the gang were taken to Malfoy Manor, where Bellatrix proceeded to torture Hermione using the Cruciatus Curse.
The Death eaters did some hideous acts and Bellatrix played a massive part in organizing and executing most of them. She followed Voldemort's instructions, regardless of how immoral or vile they may have been. Her lack of compassion and complete lack of morals and remorse sometimes make her seem worse than Voldemort.
Unlike many Death Eaters, when Bellatrix was imprisoned in Azkaban, she did not lie about her loyalty, instead proudly declaring herself to be a follower of Voldemort. Even Voldemort himself had a considerable amount of respect for Bellatrix.
He rejected his family's pure-blood elitism and reverence for the Dark Arts. At Hogwarts, rather than be sorted into Slytherin like the rest of his family, Sirius was placed in Gryffindor. In contrast to his home life, Sirius greatly enjoyed life at Hogwarts, where he was inseparable from his best friend, James Potter.
So why does Harry always use Expelliarmus? Well that would be just his natural reaction. From the wiki page harrypotter.wikia.com/wiki/Harry_Potter it says that Expelliarmus is his signature spell and would thus be his natural response to any attacks. Just like if person A trained the hammer fist a lot.
The only clear canon instance of a good wizard using Avada Kedavra to kill is Snape's euthanasia of Dumbledore.
Draco Malfoy (Cruciatus and Imperius Curses)
Although the use of these curses is rightly unforgivable, it's important to note that Draco never used them for his own benefit. It was always the pressure from Lord Voldemort that forced Draco into his crimes.
They were first classified as "Unforgivable" in 1717. They are the Killing Curse, Avada Kedavra, the Cruciatus Curse, Crucio, and the Imperius Curse, Imperio.
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.