This speculation is primarily about a character from the Fantastic Beasts and Where to Find Them films. The character is – well let me put it into the next post, to avoid it showing up as preview text.
– Nagini. In the films, AIUI, Nagini’s backstory is that she started life as an animagus of sorts. She could transfer her form between human woman and snake. But her tragedy, or doom, if you will, was that eventually, she would forever lose the ability to assume a human form.
Also, it’s established that she later teams up with Voldemort, and he uses her as the host for one of his horcruxes.
From the Harry Potter books and films, we learn (or it’s strongly implied) that the Avada Kedavra curse, when deployed against a living horcrux host, disbands the bond between the soul of the host and the soul-fragment of the horcrux’s creator. And the host doesn’t (necessarily) die.
So, if instead of bisecting Nagini, Neville (or anyone, really) had simply used Avada Kedavra on her, might that have destroyed that particular layer of protection Voldemort was using her for?
Well, I mean, bisecting her ALSO destroyed that layer of Voldemort’s protection.
And it’s not clear that Avada Kedavra would always work like that on a Horcrux-- It’s pretty strongly put forward that it only happened with Harry because of his parents’ love.
Who knows? Her backstory was created with a retcon spell, so anything could happen.
Well, maybe. Certainly his first survival if it. But contemplating it made me think the Avatar Kedavra spell might have been invented, not as a murder weapon, but a therapeutic device, intended to correct things like demonic possession. Having proven ineffective at that, it was soon turned to evil purposes.