Harry Potter and the Deathly Hallows: Part I

Further information: Harry Potter and the Deathly Hallows
See also: Plot of Part 2
The Minister of Magic, Rufus Scrimgeour, addresses the wizarding media, stating that the Ministry will remain strong even as Lord Voldemort gains strength. Harry, Ron and Hermione prepare for a journey to find and destroy Voldemort's Horcruxes, with Harry watching the Dursleys depart and Hermione wiping her parents' memories of her.
Severus Snape arrives at Malfoy Manor to inform Lord Voldemort and his Death Eaters of Harry's impending departure from No. 4 Privet Drive. Voldemort commandeers Lucius Malfoy's wand, as Voldemort's own wand cannot be used to kill Harry, because the wands are "twins."
Meanwhile, the Order of the Phoenix gather at Privet Drive and escort Harry to safety, using Polyjuice Potion to create six decoy Harrys out of everyone else. During their flight to the Weasley family home at the Burrow, all are ambushed by Death Eaters. They kill Mad-Eye Moody and Harry's owl, Hedwig, injure George Weasley, and knock out Hagrid. This forces Harry to take over and drive his flying motorbike while fighting Voldemort. In the process, destroying some power lines that causes a blackout to the city. After arriving at the Burrow, Harry has a vision of the wand-maker Ollivander being tormented by Voldemort, who claims that Ollivander had lied to him. Ollivander claimed that the only way Voldemort could kill Harry was by using another person's wand.
Scrimgeour arrives at the Burrow with Albus Dumbledore's Will and distributes three items to Ron, Hermione, and Harry. Ron receives Dumbledore's Deluminator, Hermione a copy of The Tales of Beedle the Bard, and Harry the first Golden Snitch that he ever caught in a Quidditch match. Scrimgeour reveals that Harry was also bequeathed the Sword of Godric Gryffindor. The minister states, though, that the sword was not Dumbledore's to pass on and, in any case, is missing.
Next, the Death Eaters assassinate Scrimgeour and replace him with their puppet Pius Thicknesse. Thicknesse, under the influence of the Imperius curse, begins persecuting Muggle-born witches and wizards. Then the Minister disrupts the wedding of Bill Weasley and Fleur Delacour, by sending a patronus charm to tell everyone what they did and attack them. Harry, Hermione, and Ron Disapparate to London and find sanctuary at No. 12 Grimmauld Place. There they discover that the "R.A.B." from the false Horcrux locket is Regulus Arcturus Black, the younger brother of Sirius Black. Kreacher, the Black's house-elf, tells them that Mundungus Fletcher stole the real locket. Kreacher and Dobby apprehend Fletcher, who reveals that the locket is in the possession of Dolores Umbridge. Under the disguise of Polyjuice Potion, the trio infiltrate the Ministry. In the courtroom, Umbridge is interrogating a Muggle-born witch and threatens her, which enrages Harry, who says to her "You're lying, Dolores. And one mustn't tell lies." He then stuns her and Hermione successfully retrieves the locket. The trio escape from the Dementors by fleeing into the wilderness after accidentally revealing the location of No. 12 Grimmauld Place to Yaxley, a Death Eater.
Unable to destroy the Horcrux, they take turns wearing it to dilute its malignant power. Harry sees a vision of Voldemort interrogating Gregorovitch, a renowned wand-maker, who claims that a teenage boy had once stolen the legendary Elder Wand from his shop. While Ron wears the locket, he is overcome by his negative feelings and after arguing with Harry, he leaves, upsetting Hermione. Harry and Hermione decide to go to Godric's Hollow, where they visit Harry's parents' graves and the house where they were killed. Next Harry and Hermoine visit Bathilda Bagshot, who they believe has the Sword of Gryffindor, which they deduce can destroy Horcruxes. They are surprised by Voldemort's snake, Nagini, who has been possessing her corpse. Hermione accidentally breaks Harry's wand as they escape into the Forest of Dean. Hermione is able to identify the mysterious thief seen in Harry's vision as Gellert Grindelwald.
When evening falls, Harry sees a Patronus in the form of a doe which leads him to a frozen pond. Gryffindor's sword lies beneath the pond's ice. Harry breaks the ice and jumps in to reach the sword. The locket around his neck attempts to strangle him, but Ron arrives to rescue Harry. Despite the soul inside the locket attacking both of them, Ron manages to destroy it with the Sword of Gryffindor. Hermione is angry with Ron at first but then reconciles with him.
The trio then visit Xenophilius Lovegood to learn about a symbol seen several times on their journey. He tells them that the symbol represents the Deathly Hallows: the Elder Wand, the Resurrection Stone and the Cloak of Invisibility. Lovegood betrays them to the Death Eaters in an effort to have his kidnapped daughter returned.
The trio escape into the wilderness once more, but Snatchers appear and chase them. Harry has a vision of Voldemort learning from Grindelwald that the Elder Wand lies with Dumbledore in his grave. They are then captured and taken to Malfoy Manor. Bellatrix Lestrange imprisons Harry and Ron in a cellar, in which they discover Luna, Ollivander, and Griphook the goblin. Bellatrix tortures Hermione for information on how they got the sword of Gryffindor. After Dobby apparates in the cellar to save them, and incapacitates Peter Pettigrew, a short battle ensues; Harry duels and disarms Draco Malfoy. Narcissa Malfoy's wand is taken by Dobby who says "Dobby has no master. Dobby is a free elf. And Dobby has come to save Harry Potter and his friends." Dobby is killed by Bellatrix right after he helps Harry and the other captives escape. Meanwhile, Voldemort breaks into Dumbledore's tomb and takes the Elder Wand.

Question: Who killed Dobby?
Answer: Bellatrix