In the movie Harry Potter and the Deathly Hallows, Ron tries to use magic to kill Nagini, but fails. In the movie, Neville Long bottom succeeds in killing Nagini with the Sword of Gryffindor. Could Avada Kedavra have killed Nagini? Avada Kedavra was used kill Harry, a living Horcrux, so could Avada Kedavra have killed Nagini, also a living Horcrux?
We can't know.
Since we know that Harry wasn't a true Horcrux, his vulnerability to avada kadavra can't be used to assume that Nagini was also vulnerable to the killing curse.
The Twitter comments from JKR about a living "Horcrux-receptacle" needing to be destroyed beyond repair does not at all talk about avada kedavra. Rather, it's in reference to basilisk venom, which is already known as a something that can destroy a Horcrux. In Nagini's case, we know her Horcrux status was technically destroyed by basilisk venom, which had been absorbed into Gryffindor's sword.
I also think JKR's Twitter comments are less clear than they could or should be. For instance, she uses the term "Horcrux-receptacle" when she's made it clear that Harry is not a Horcrux, made with neither the intent nor magic necessary to make a Horcrux. To me, this reads as a hypothetical situation: "If Harry were a Horcrux, he'd have to be destroyed beyond repair and killed." However, JKR used very unclear verbiage in her tweet:
- If Harry were a "Horcrux-receptacle", she's previously made it clear that Harry wasn't a Horcrux, and so doesn't have the protective enchantments that keep most other magic from harming him. So, basilisk venom, as presented in the question to JKR, should have no role regarding the Voldemort soul splinter in Harry.
- What is a "Horcrux-receptacle", anyway? JKR defined a Horcrux as 'the receptacle in which a Dark wizard has hidden a fragment of his soul for the purposes of attaining immortality." But, her wording in the noted tweet implies that the Horcrux is the split soul, and not the receptacle. Otherwise, she was calling it a "Receptacle receptacle"?
The "destroyed beyond repair" bit even confused followers, who asked:
but what about the resurrection stone from the ring that still worked even after its horcrux was destroyed?
To which she replied:
The crack in the stone was irreparable. Only Dumbledore cd have extracted the soul fragment but left the original charm intact.
So, the original charm of the ring (some incredibly powerful, unique magic), was able to be left intact, but a "crack" in the ring made it considered to be beyond repair. While it's from the mouth of JKR, I find this explanation dubious at best; an overly convenient way of explaining her oversight from the previous tweet. This explanation further clouds the issue:
- The destruction of a Horcrux-enchantment is not the same as the destruction of the Horcrux-object (the Hallow ring persists, charm intact).
- Destruction of the Horcrux-object also destroys the Horcrux-enchantment (Nagini is killed, the Horcrux and soul piece and destroyed)
While the killing curse is touted as unblockable, it is now just as often said that only one known counter exists, which is the love-based sacrifice charm that protected Harry. Yet, a living Horcrux may not be a valid target of the curse, anymore than a statue would be, but that would be unknown. We have no examples to show that a living Horcrux would actually be vulnerable to avada kedavra.
We have no examples of avada kedavra being used to permanently kill any notably magical species. The closest we come to that is Fawkes blocking the killing curse meant for Dumbledore, but Fawkes is immediately reborn (and apparently keeps his memories, or at the very least his bond to Dumbledore). So, here we have precedent that highly magical creatures may circumvent avada kedavra, due to their own natural abilities.
- There are no other examples of living Horcruxes in the Harry Potter universe
- There are no examples of any living or inanimate Horcrux ever being targeted by avada kedavra
- There are plenty of examples of Horcruxes being immune to many powerful spells
- There is at least one known instance where a living being with part of Voldemort's soul in it being able to live after being hit with avada kedavra (that instance being Harry Potter, when the Voldemort soul splinter was also finally removed)
- A living Horcrux has to be to destroyed beyond repair, which death doesn't necessarily do (see Fawkes)
- We only have one example of something able to kill a living Horcrux, and that substance (basilisk venom) was used to used to destroy all the other Horcruxes excepting one, which was destroyed by Fiendfyre.
- The nature and origin of Nagini has been left unclear by JKR, so we don't know if she has any ability, like phoenixes, that would allow her to overcome death
- We don't know about the exact nature of the protections being a Horcrux brings. When most wizards say nothing stops avada kedavra, we can be reasonably sure none of them have knowledge of whether that's true against Horcruxes, if they even know what a Horcrux is to begin with.
With these things in mind, there are too many ifs buts and maybes, so we can't form a definitive conclusion about Nagini's post-Horcrux vulnerability to avada kedavra. Perhaps we'll find out someday through Pottermore or better-worded tweets.
JK Rowling has made a comment to this effect (emphasis mine):
The Horcrux-receptacle has to be destroyed BEYOND REPAIR, so Harry would need to have DIED.
To me, this quote indicates that to destroy a horcrux, irrespective of whether it is living or not, it must be destroyed beyond repair. In the case of a living being, once it is dead it cannot return to life - even the resurrection stone doesn't quite fulfill this. As pointed out in chat by @Slytherincess, considering that, as per Barty Crouch Jr (in the form of Mad Eye Moody) explained that avada kedavra cannot be deflected or protected against, this indicates that, despite being a horcrux, Nagini was still susceptible to that particular curse.
The canon sources do not suggest that Nagini is invulnerable to Avada Kedavra. So yes the Killing curse should be enough to destroy the Horcrux in Nagini, as that would destroy her beyond repair.
Harry, at least, thought that Nagini could be killed with spells.
Right before Harry gives himself up to die he tells Neville that Nagini needs to be killed:
"No," Harry lied easily. "'Course not... this is something else. But I might be out of sight for a while. You know Voldemort's snake, Neville? He's got a huge snake... calls it Nagini..."
"I've heard, yeah... what about it?"
"It's got to be killed. Ron and Hermione know that, but just in case they –"
Harry tells this to Neville because he needs to make sure enough people know about it in case some of them die. If Nagini could only be killed in a specific way (e.g. with Gryfindor's sword) that would be a really important point for Neville to know. Presumably, then, if Harry did not tell him this, he must have thought that Nagini could be killed naturally with magic.
Earlier when Harry is in the Shrieking Shack with Voldemort, he considers trying to kill Nagini:
Snape strode past the gap, and Harry drew back a little, keeping his eyes fixed upon Nagini, wondering whether there was any spell that might penetrate the protection surrounding her, but he could not think of anything. One failed attempt, and he would give away his position...
Harry refrains from attacking Nagini because he doesn't know what spell could break through the cage. The implication is that if he had known a spell to break through the cage he would have done so, and attacked Nagini. He only had his wand with him at the time, so he must have been planning to use a regular spell to kill her. Clearly, then, he assumed it would be possible to kill Nagini with a spell.
Later when he enters Voldemort's base in the Forbidden Forest, Harry again contemplates attacking Nagini:
Harry could feel his wand against his chest, but he made no attempt to draw it. He knew that the snake was too well protected, knew that if he managed to point the wand at Nagini, fifty curses would hit him first.
Here again Harry refrained from attacking Nagini for an external reason, namely, that he assumed the Death Eaters would stop him. The implication again is that he would have attacked Nagini with a spell if not for the external factor. Thus we see again that he believed Nagini could be killed with a spell.