Warning: Spoilers for Fantastic Beasts: The Secrets of Dumbledore.
Fantastic Beasts: The Secrets of Dumbledore deepens the mythology of the Wizarding World movies, but at the same time also retcons the rules of the Avada Kedavra killing curse. The Harry Potter movies and books have a well-established set of rules that the filmmakers take great care to stay true to. While there are instances of the Harry Potter rules being bent previously in service of the story, Fantastic Beasts 3 takes this a step further by retconning the rules for one of the most powerful spells in the Wizarding World.
In the world of Harry Potter, the killing curse, Avada Kedavra, is dark magic, and one of the three unforgivable curses (the others being the Imperius (loss of free will), and Cruciatus (torture)). When cast successfully, Avada Kedavra causes instantaneous death, with the punishment for the witch or wizard using it being life imprisonment in Azkaban Prison. Both the Harry Potter and Fantastic Beasts movies have used the killing curse, and Harry’s survival of it and the resultant scar is what makes him so famous in the Wizarding World. The killing curse crops up twice in Fantastic Beasts 3, and in both cases, the movie retcons part of the established rules of Avada Kedavra.
At the start of Fantastic Beasts: The Secrets of Dumbledore, magizoologist Newt Scamander is assisting a Qilin (a magical beast from Chinese mythology) give birth when they are attacked by Grindelwald’s forces, and the mother is hit twice with the killing curse. The mother Qilin doesn’t die immediately, though, and there is a whole chase sequence with Newt and the newborn Qilin that follows while the mother is still alive. Both the Harry Potter movies and books have previously established that the Avada Kedavra curse should cause instant death, so there shouldn’t be a need to cast a second curse, let alone have it still result in a protracted demise. Another established rule that Fantastic Beasts 3 retcons is that the killing curse is supposed to be unblockable by any typical protective spell. 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.
There are a couple of possible explanations for the retconning of the killing curse rules in Fantastic Beasts: The Secrets of Dumbledore. The Qilin has extraordinary magic abilities, so much so that the Wizarding World places its blind faith in its magic to select its leaders. The sheer amount of rare magical power that the beast processes could potentially delay the effects of the Avada Kedavra curse, albeit not prevent the outcome. In regard to Albus and Aberforth blocking Grindelwald’s killing curse at the end of Fantastic Beasts 3, this could be down to the blood pack itself. Though Albus isn’t sure why the blood pack breaks, he says to Newt afterward, “he sought to kill, I sought to protect.” While the killing curse is in essence abject hatred, the Dumbledore brother’s protection spells stem from love. The polarity between these two when connected could have surged into the blood pack rather than onto Credence. The power of love as a defense against evil also echoes Lilly Potter’s sacrifice to save baby Harry from Voldemort – though in that case, it cost Lilly her life.
Ultimately, regardless of in-universe explanations for retconning the killing curse, the real reason is likely to be down to the filmmakers serving the plot of Fantastic Beasts: The Secrets of Dumbledore. The drawn-out death of the Qilin is heart-breaking, but it gives Newt a character beat to comfort her in the last moments, as well as discover the second baby Qilin. The blocking of Grindelwald’s Avada Kedavra curse meanwhile facilitates the destruction of the blood pack in a more satisfying and cinematic way than watching Dumbledore’s trial and error of spells back at Hogwarts. Notwithstanding this, the retcon of the killing curse does, unfortunately, dilute some of its menace and power.
