Protecting Harry Dumbledore does not use any language to make one think that Nagini is anything more than a snake. But this could be despite his knowledge of Nagini's past because he has to factor his grand plans into everything he tells (or doesn't tell) Harry.
It wasn't just inanimate objects that could become Horcruxes. Lord Voldemort's beloved snake companion, Nagini, was also transformed into one, and Dumbledore seemed to know it, ominously predicting that a day would come where Voldemort would 'fear for his snake'.
Lord Voldemort and Nagini had a special relationship
As a Parselmouth, (i.e. able to talk to snakes) Voldemort could communicate with her, and the pair often sent each other messages, even suggesting a telepathic connection at times.
Nagini is loyal to Voldemort because he can speak with snakes. As a Maledictus, there probably isn't much time after the events in The Crimes of Grindelwald before Nagini falls to her blood curse, becoming a snake forever.
In 1927, she was a featured attraction in a Wizard circus owned by Skender, where she became friends with Credence Barebone. She helped him learn who he was and accompanied him to Grindelwald's meeting where he revealed his plans.
When Harry and Hermione are on the first floor of Bathilda's house, Nagini (inside the corpse of Bathilda) tells Harry to "Come!" from the next room in Parseltongue. In reaction, Hermione jumps and clutches Harry's arm, and the two of them obey the command.
She's genuinely there for him every step of the way, and that means a lot to him. The Crimes of Grindelwald's climax sees Credence leave Nagini behind to join Grindelwald and learn his true identity. She then appears alongside Newt and his allies, implying she'll take their side in the upcoming war against Grindelwald.
But now we know Nagini was no simple supervillainous pet, but a human woman cursed with the “Maledictus” blood curse, which, according to Rowling, is a female-only inherited condition which grants the affected individual the ability to transform into an animal at will, until one day, the transformation becomes ...
He was afraid that the Elder Wand, recognizing his true owner, wouldn't work as intended, or worse, backfire on him. Giving Nagini the order to kill, indirectly would have led him to win the Wand's allegiance as per him, without any side-effects.
The last time he sees her, she's with Voldemort and he has to play dead, so no talking there. Plus, she has to die to defeat Voldemort.
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.
Nagini, Voldemort's snake, then eats Professor Burbage's dead body. A story about her "resignation" from Hogwarts appears in the Prophet, but members of the Order of the Phoenix discount it, as they are unable to locate her anywhere after that.
In the sequel, she decides to join Gellert Grindelwald in his crusade on the promise that she can marry Jacob due to a law refusing to allow humans and witches to marry. However, Queenie turns to the side of good again and both she and Jacob marry each other in 1932.
Nagini the Snake
Nagini was the last horcrux to be destroyed and also the last to be made. It was created when Peter Pettigrew bumped into Ministry of Magic employee Bertha Jorkins in Albania while Voldemort was still weak.
Harry Potter and the Deathly Hallows
In this book, Rowling reveals that Bellatrix is the guardian of Helga Hufflepuff's cup (though she is unaware that it is a Horcrux), which Voldemort has entrusted the Lestranges to keep in their Gringotts vault.
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.
In a film extra called "Credence, Nagini and the Circus Arcanus," available on the "Crimes of Grindelwald" Blu-ray, Rowling said she "knew all along about Nagini." "Now, there were always hints that she had been human," Rowling said. "In her name — because the Naga are, in mythology, a race of snake beings.
Why didn t the Death Eaters use Avada Kedavra all the time? Firstly according to the fake Moody Avada Kedavra (AK) requires a powerful magic. Therefore using it repeatedly would be magically - and perhaps even physically - very draining. Secondly Death Eaters don't always want/need to kill their targets.
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.
Fact: Nagini Is A Maledictus
A Maledictus, according to good ol' J.K., is a witch who has been born with a blood curse, carried on by her mother.
While it's understandable Nagini is not in The Secrets of Dumbledore, there are plenty of stories left to tell regarding Voldemort's most beloved Horcrux. But her origins may never come to light, given the third Fantastic Beasts movie's mixed reception.
There was a rumour that Nagini was the snake Harry released from the zoo in Harry Potter and the Philosopher's Stone. This was proven false, as Nagini is some type of viper while the snake in the zoo was a boa constrictor. Furthermore, the snake was given a male voice in the film, while Nagini is female.
Later, Newt explains to older brother Theseus that Tina has been promoted to Head Auror for MACUSA. Her new role is why she cannot join Newt and Dumbledore on their mission to thwart Grindelwald's plans to be elected to the International Confederation of Wizards.
Though it may later be revealed that the Dark Lord did indeed know that Nagini was a Maledictus, there's currently no indication that he (or anyone else) ever thought that she was anything other than a snake who was made into one of the Dark Lord's Horcruxes.
The only "emotion" Voldemort felt when Nagini died was fear because she was the second to last piece of his soul. If Nagini wasn't a horcrux, he wouldn't even have kept her around.