Voldemort concludes that he is not yet the true master of the Elder Wand, because Snape killed its previous owner, and thus Voldemort must kill Snape to be the wand's master. Voldemort orders Nagini to kill Snape, and the great snake bites Snape's neck, mortally wounding him.
He concludes that, because Snape killed Dumbledore, the Elder Wand belongs to him. So, Voldemort had to kill Snape for the wand's allegiance to switch. Voldemort knew he couldn't cast Avada Kedavra with the Elder Wand to kill its master, so he ordered Nagini to be the one who killed Snape.
He uses his snake, Nagini, to kill Snape rather than any number of spells he knows. This allows Snape to live long enough to help Harry defeat him.
Nagini seemed integral to Lord Voldemort's survival before he could be restored to his body, and we learnt, quite grotesquely, that Voldemort even tasked Wormtail to 'milk' Nagini so he could live off her venom to keep his strength up. BFFs forever.
Nonetheless, despite her initially benevolent nature, Nagini's blood curse eventually turned her into a snake permanently. Though it is unknown if she became evil as a result of that, or was turned evil as a result of Voldemort turning her into a Horcrux. As a snake, Nagini was completely loyal to Lord Voldemort.
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.
By 1994, Nagini was seemingly trapped in snake form and belonged to Lord Voldemort, with whom she had a special bond largely due to becoming a Horcrux after her master had killed Bertha Jorkins in 1994.
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.
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.
Therefore, when Voldemort used the killing curse against Harry in Deathly Hallows - Part 1, he was actually killing part of his own soul, and Harry was able to survive. And after Neville killed Nagini, all Horcruxes were destroyed.
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'.
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.
Deathly HallowsEdit
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.
Coming across Arthur Weasley -- a member of the titular Order of the Phoenix and the patriarch of the Weasley family -- Nagini attacked, coming close to killing the man. However, thanks to Harry informing Dumbledore of the attack, other members of the Order were able to reach Arthur in time to save his life.
Quirrell didn't use the Killing Curse on Harry, because Voldemort said “use the boy” to get the Sorcerer's stone from the Mirror of Erised. If Quirrell had killed Harry right away, he would have lost the chance to get the stone.
As an unqualified wizard who didn't finish his education, Hagrid was not technically allowed to perform magic.
He always seemed like he truly loved Nagini. He must have felt something for Nagini or he wouldn't have given her a part of his soul.
Behind the scenes
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.
Rowling previously revealed that Nagini is a Maledictus, which means she carries a blood curse that will doom her to transform permanently into a beast. 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."
In the HP books, Nagini is a large, deadly and somewhat sentient snake. In the expanded wizarding world we discover Nagini is a maledictus.
7 horcruxes were diary, ring, locket, cup, tiara, Nagini and Harry. They were destroyed by Harry, Dumbledore, Ron, Hermione, Crabbe, Neville and Voldemort respectively. Amongst them, only Neville and Crabbe didn't know what Horcrux actually were.
Funnily enough, the most famous Lestrange is one only by marriage, and despite really adorable Instagram photos that make us wish it so, there is no blood relation between Leta and Bellatrix whatsoever. At most, Leta and Rodolphus Lestrange (Bellatrix's husband) are very distant cousins.
Once he learned that his horcruxes were being hunted, he knew Nagini was at risk. He couldn't know how his enemies had been able to find his other horcruxes, but he had to suspect at the least the possibility that it was by some magical means unknown to him, perhaps devised by Dumbledore.
It is explained that Credence is the illegitimate son of Albus's younger brother, Aberforth (Richard Coyle). His real name is Aurelius Dumbledore.