Nagini is the last Horcrux that Voldemort created and the last one to be destroyed. During the final battle of Hogwarts, Neville pulls the sword of Godric Gryffindor from the Sorting Hat and beheads Nagini, destroying the last Horcrux.
Harry avoids encountering any of his other friends, but goes up to Neville and gives him the information that the snake Nagini must be destroyed, in case anything happens to Ron and Hermione. So if the chance presents itself, and Neville happens to think of it . . . he should kill the snake.
Voldemort then decided that it was no longer safe to send Nagini on any more missions for him. Shortly before the one-hour armistice during the Battle of Hogwarts, Voldemort used Nagini to murder Severus Snape in the Shrieking Shack. He did this by expanding the cage over and on top of him.
Neville retrieved it from the Sorting Hat just as Harry had, so perhaps this theory is correct in saying that whenever a true Gryffindor has need of the sword it appears (as Dumbledore said to Harry after the Chamber of Secrets debacle).
In the book, Neville kills Nagini, plain and simple. Neville is the unsung hero and he's the only one who ever attempts to kill Nagini because Harry tells him to.
Voldemort also had the choice of Neville Longbottom, but he chooses Harry to kill because Harry is a Half-Blood, like Voldemort, where as Neville was a Pure-Blood. Voldemort saw Harry as a greater risk that Neville.
In 1998, Fred participated in the Battle of Hogwarts, defending the castle's passageways with his twin. While fighting alongside his brother, Percy, he was killed in an explosion possibly caused by Death Eater Augustus Rookwood.
In Harry Potter and the Order of the Phoenix, our beloved Weasley patriarch Arthur suffered a brutal attack from Nagini – Lord Voldemort's snake companion – while guarding the Department of Mysteries; an attack that seemed like it could be his end.
He was given a Blood-Replenishing Potion, as he continued to bleed profusely any time his bandages were removed, suggesting that Nagini's venom somehow prevented or delayed blood clotting.
A Maledictus (plural Maledictuses) was a female individual who carried a blood curse that eventually turned her into a type of beast permanently.
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.
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.
In short, no one knows exactly how Nagini ends up as Voldemort's closest ally. Evidently, something happened between the time of her joining Newt and the events of Harry Potter that pushed her to the dark side. She might even have met Voldemort while still human, and he was Tom Riddle.
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.
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.
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.
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.
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.
Lupin, played in the films by David Thewlis, was murdered in the battle by Death Eater Antonin Dolohov, while Tonks was killed by Bellatrix Lestrange, leaving their son, Teddy, an orphan. Lupin's death is a sore spot for many fans, who fell in love with the werewolf, nicknamed Moony.
Weasley family patriarch Arthur was placed under the Imperius curse by Voldemort and his Death Eaters during the Dark Lord's rise to power, making him one of the few legitimate victims of the curse in a world where most of those who claimed they had been bewitched were just doing it to justify the actions they took ...
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.
Harry, returning to the school, looks into the Great Hall and sees Lupin and Tonks lying among the dead. While this is not mentioned in the book, the author has mentioned in an interview since its publication that Tonks was killed by her aunt Bellatrix Lestrange.
Ariana died when she was accidentally struck by a curse in a three-way duel between her older brothers and Dark Wizard Gellert Grindelwald, Albus's friend at the time; it was never known who struck the killing blow. This event would have a profound impact on both her brothers' lives.
According to the films, Hedwig's killer might have been Selwyn, who was known to be pursuing Harry and Hagrid at the time since Hedwig's death in the film happened quite a while into the fight when there was only a couple death eaters following them, or it might have been Stanley Shunpike under the Imperius Curse.