We know that they did not use Avada Kedavra.
Molly took Bellatrix out with one spell. How, exactly, is using the Killing Curse (which kills instantly) not as 'nice' as using a spell that still killed her but could have made it far more painful?
Known uses
Molly used this curse during the Skirmish at the Great Hall on Bellatrix in a fierce duel, which killed her.
But when Ron and Hermione are about to die by Nagini, Ron casts the killing curse as a last resort and it simply rebounds off Nagini as it was nothing.
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.
The darkest of the Dark Arts, Harry Potter unforgivable curses are the Cruciatus Curse, the Imperius Curse, and the Killing Curse. Although the curses are highly illegal, Harry using Unforgivable Curses happened, but he uses them once he was thrust into the Second Wizarding War.
Draco Malfoy (Cruciatus and Imperius Curses)
He's never seen using the Killing Curse, but under strict orders from Voldemort, he's known to have used the other two.
Harry can no longer speak it
Harry being an accidental Horcrux meant he was bound to Voldemort in so many ways, just like Voldemort was bound to serpents. Not only could Harry speak the language of the snake, but could see through the eyes of Nagini, another of Voldemort's Horcruxes, as it turned out.
In the film Harry Potter and the Deathly Hallows: Part 2, this duel has some noticeable differences: In the film, Bellatrix casts a blue spell at Ginny Weasley instead of a Killing Curse, who deflects it using a Shield Charm.
The Killing Curse ('Avada Kedavra')
Obviously there's no coming back from this spell, and according to the Ministry of Magic's classifications (and, again, fake-Moody) this is the worst Unforgivable Curse.
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.
5 Bellatrix Lestrange: The Cruciatus Curse.
Voldemort intentionally made six Horcruxes, but when he used Avada Kedavra on Harry, he unintentionally created a seventh Horcrux. Instead of dying, Lily's love for Harry created a counter 'curse' known as Sacrificial Protection and saved Harry.
The only clear canon instance of a good wizard using Avada Kedavra to kill is Snape's euthanasia of Dumbledore.
You won't be able to unlock the spell until very late on in Hogwarts Legacy however, so if you're hoping you can use it to make the game's bosses a piece of cake while running through the main story missions, you might have to think again. It comes so late in the game that you might have already completed it.
In Harry Potter lore, the Avada Kedavra is accompanied by a blinding flash or jet of green light, and can only be countered by sacrificial protection, which used the power of love. But that's not going to do you much luck in Hogwarts Legacy.
And the third, the Killing Curse, is named by Hermione; Moody mentions that it can't be blocked, and only one person is known to have ever survived it: Harry Potter. Harry makes use of two of the Unforgivable Curses in the books.
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.
In Cursed Child – Part Two, Draco Malfoy explains to Harry that his wife, Astoria Greengrass, was sick for many years. Draco says she suffers from a “blood malediction,” which ultimately ends up killing her the summer before Scorpius's third year.
The three Unforgivable Curses in Harry Potter are Crucio, Imperio, and Avada Kedavra, and all three are available to Hogwarts Legacy players. Crucio subjects the victim to extreme torture, while Imperio puts them under the control of the spellcaster.
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.
Harry became so enamoured with the spell that he eventually used it to finish off Lord Voldemort. Here's how Harry's relationship with Expelliarmus became his signature – and why that's a good and bad thing.