Expelliarmus did not beat Avada Kedavra. Voldemort's wand (the elder wand) recognized its true master (that was Harry) and refused to attack it. In a way Voldemort killed himself as his own spell backfired on him.
During his final confrontation with Voldemort, Harry knew that he (Harry) was the Elder Wand's true master. He again cast Expelliarmus, ejecting the wand from Voldemort's hand into his own as the Killing Curse rebounded off his body, fatally striking Voldemort.
Defeating The Dark Lord with his signature spell
Disarming is about defence, not murder. Voldemort was a killer, Harry was not. It was an important distinction for Harry, who had learned so much about Voldemort and seen first-hand the ways in which they were similar.
The disarming spell makes the witch or wizard lose his or her wand which is an extension of the witch or wizard themselves. This leaves the person defenceless and negates them as a threat. It's a pretty good spell, even if it isn't as sophisticated as other curses and jinxes.
[Discussion] Expelliarmus is the most powerful spell in the wizarding world.
Its incantation is Avada Kedavra. The only known counter-spell is sacrificial protection, which uses the magic of love. However, one may dodge the green bolt or block it with a physical barrier.
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.
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.
History Of Avada Kedavra
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.
At the end of Fantastic Beasts 3 though, Grindelwald attempts to use Avada Kedavra on Credence, but Albus and Aberforth Dumbledore successfully block it using golden magic.
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.
Avada Kedavra
It's used to kill people, it's an Unforgivable Curse, and it was Voldemort's favourite, so it's going last.
Firestorm. Firestorm was a spell that created a large ring of fire around the caster's wand. This spell is one of the last and most powerful spells that Albus Dumbledore cast using the elder wand before his death.
The Disarming Charm became Harry Potter's signature spell, and it notably caused the death 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 that ...
Alohomora, which is a spell that unlocks objects (and it's also Hermine Granger's signature spell).
Those that are gifted with the power of flight are perhaps more rare than any other wizard that has any other power. Able to fly purely because of their own powers and not reliant on any type of enchanted item or spell at all, the number of these special wizards known to exist can be counted on one hand.
I've always suspected Molly's lethal spell was a particularly well-placed and powerful stunner that stops Bellatrix's heart, which is consistent with the description of her death: Molly's curse soared beneath Bellatrix's outstretched arm and hit her squarely in the chest, directly over her heart.
Quirrell didn't use the Killing Curse on Harry, because Voldemort said “use the boy” to get the Sorcerer's stone from the Mirror of Erised. If Quirrell had killed Harry right away, he would have lost the chance to get the stone.
Malfoy spots Harry and casts a Cruciatus curse; Harry, defending himself, using the Half-Blood Prince's Sectumsempra spell without knowing its effects. To his horror, it gashes Draco's face and chest, spilling his blood everywhere. Moaning Myrtle flies off, screaming.
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.
At the beginning of this book, he measures it against Lucius Malfoy's showing he still has it and it is intact. Dumbledore explains to Harry in book 4 that since both wands contained a tail feather from Fawkes the phoenix, the couldn't face each other as they'd refuse to destroy each other pretty much.
In Harry Potter And The Deathly Hallows Part 2, Voldemort kills Snape using his snake Nagini instead of his favorite spell Avada Kedavra - here's why. Voldemort killed Snape in Harry Potter And The Deathly Hallows Part 2 using his Horcrux snake Nagini instead of his favorite Unforgivable Curse, Avada Kedavra.
Draco Malfoy's mother Narcissa was cold, cunning and devoted to the Dark Lord. But she was also a mother, which meant she was willing to risk everything to make sure her son was safe. When Harry survived Voldemort's Killing Curse for the second time, Narcissa pretended he was dead so she could get to Draco.