Voldemort summons Snape to the Shrieking Shack. Erroneously believing Snape is the master of the Elder Wand and that Snape's death will make him the master of the Wand, Voldemort kills Snape by having his pet snake Nagini bite him through ...
In the belief that the Elder Wand will grant its allegiance only to one who has slain its previous owner, Lord Voldemort has his pet snake, Nagini, kill Severus Snape.
Voldemort orders Nagini to kill Snape, and the great snake bites Snape's neck, mortally wounding him.
Why did Voldemort have Nagini kill Snape instead of doing it himself? Because he thought he couldn't do it himself. Voldemort was holding the Elder Wand, but thought that its true allegiance laid with Snape, as he was the one who had killed Dumbledore, its previous owner.
If Snape did defend himself, that would have jeopardized a major part of Dumbledore's and Snape's plan of preventing Voldemort from earning the "allegiance" of the Elder Wand. In the act of defending himself, Snape would have raised Voldemort's suspicions.
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.
Voldemort possesses Nagini because she's immortal.
YouTuber Seamus Gorman points out that Nagini's Maledictus curse could make her immortal, and we all know that Voldemort's thirst for power would have loved that.
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. Burbage is later replaced as Muggle Studies teacher by Alecto Carrow.
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'.
Unless one or the other has a significantly smaller role in the films than we are led to believe, Dumbledore and Nagini will get to know each other. From the very little we see of Nagini, it looks like she's at least potentially on the same side as Dumbledore.
So it's possible that Nagini met Tom Riddle in his early years of rising to power but by the time she found him in Albania she had been cursed to become a snake entirely, and losing the ability to turn back into a human.
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.
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. In reality, the Elder Wand belonged to Draco, as he was the one to disarm Dumbledore in the Astronomy Tower.
In order to conjure the avada kedavra curse, you have to want to kill your victim. We all know that Voldemort could easily kill a child without an ounce of remorse... but not Snape. Snape didn't want to kill Dumbledore, and this was why the spell was blue instead of the usual green.
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.
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.
This means that the killing curse (if I'm correct) has the power to destroy horcruxes. But when Ron and Hermione are about to die by Nagini, Ron casts the killing curse as a last resort and it simply rebounds off Nagini as it was nothing.
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.
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.
Charity Burbage floats helplessly above a dinner table, as if paralysed. She begs Snape to help her. Voldemort point his wand at her, and a green light erupts out of its end. The light hits Charity, and she falls down on to the table dead, her eyes open and her face splattered with droplets of blood.
Isaacs brings up Lucius' disheveled appearance in Deathly Hallows, raising the possibility that the Malfoy patriarch had begun drinking too much. The way Isaacs saw it, Lucius was in a no-win situation. Given everything he'd done, the man no longer had a place on either side of the war.
The Deathly Hallows Part I begins with a whole bunch of double-crossing and betrayals. Snape betrays the Order of the Phoenix. A guy from the Ministry of Magic becomes a Death Eater, which is probably how the Ministry ends up falling. And that's just the beginning of the movie.
In order to get her venom, Pettigrew had to milk Nagini for it. However, milking a snake is done via the fangs by having them bite a latex cover over a glass jar, then stimulating the venom sacks by either manually massaging them or using electricity to make the muscles contract.
The second film in the series, written by Harry Potter architect, J.K. Rowling, revealed in a trailer that fans had been on the right trail. Nagini, it turns out, was actually a Maledictus—a witch with a blood curse that turns her permanently into a beast (in her case, a giant snake).
Voldemort was born Tom Riddle in 1926, which is when the first Fantastic Beasts film takes place. In Fantastic Beasts: The Secrets of Dumbledore, there's a bit of a time jump, setting the year as 1932. This would make Tom Riddle around 6 years old at the time.