Hedwig's killer is not explicitly identified, so he/she may have appeared in other books, films or video games, but the only known appearance is
The theory surfaced on Tumblr, claiming that Snape was actually the Death Eater who killed Hedwig in order to keep the other Death Eaters from identifying Harry as the real one.
She was struck down by a killing curse when Harry and his friends — who were all disguised as Harrys thanks to the polyjuice potion — tried to leave Privet Drive at night, only to be tailed by a group of masked Death Eaters.
Hedwig's death was as a result of a Killing Curse cast by a random Death Eater. There is no way of knowing who killed her because there were curses flying everywhere, and Harry and Hagrid were surrounded.
In 1995, Hedwig was attacked by associates of Dolores Umbridge in an effort to intercept Harry's mail, requiring Dobby and Professor Grubby-Plank to nurse her back to health.
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.
Moody's death came at the betrayal of Mundungus, and the moment he disapparated, Moody was struck by a killing curse by Voldemort and fell off his broom to a certain death.
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.
Both Lupin and Tonks die in combat, killed by Dolohov and Bellatrix Lestrange, respectively, leaving Teddy an orphan with Harry Potter as his godfather and Andromeda Tonks as his guardian. JK Rowling has since stated that she originally intended for both Lupin and Tonks to survive.
In the book, Hedwig was killed as she sat in her cage by Harry's side – as she had so many times before – as they attempted to escape on the back of Hagrid's motorcycle. In the film, the scene was made even more heartbreaking, as Hedwig was killed while attempting to protect Harry from Death Eaters.
Hedwig was shot out of the sky, and Harry got absolutely zero seconds to react to it before he had to continue fighting for his own life, even having to destroy the side car with her body. There wasn't much consolation afterwards either, in my opinion.
Hedwig's killer is not explicitly identified, so he/she may have appeared in other books, films or video games, but the only known appearance is Harry Potter and the Deathly Hallows and Harry Potter and the Deathly Hallows: Part 1. A popular fan theory based on book canon suggests that Snape may have killed Hedwig.
"Hagrid is one of Voldemort's oldest associates, and knows his true identity" In Chamber of Secrets, it is revealed that Hagrid had been a student at Hogwarts during the same period that Tom Riddle (the true identity of Lord Voldemort) also frequented the school.
The Death Eaters and the Dark Lord don't simply cast Avada Kedavra on everyone all the time because it doesn't suit their long-term goal of ruling the world and having a wizard-run society.
Fortunately, the movie version of Harry Potter and the Deathly Hallows, Part 1 addresses my concerns perfectly. Spoiler: Hedwig still dies, but Harry had set her free from her cage before he flew off, and she came back to save his life.
Despite his attraction to the Dark Arts and Voldemort's ideology of wizard supremacy, Snape's love for Muggle-born Lily Evans, Harry's mother, eventually compelled him to defect from the Death Eaters. He then became a double agent for Albus Dumbledore and the Order of the Phoenix.
Colin was expelled from Hogwarts in his sixth year, due to Voldemort's policy of not allowing Muggle-borns to attend the school. On 2 May, 1998, he snuck into the Room of Requirement with the rest of Dumbledore's Army and fought in the Battle of Hogwarts, during which he was killed by Yaxley.
Pettigrew attempted to strangle Harry, but momentarily slackened his grip when Harry reminded him of his life debt, and was killed for it by the silver hand made for him by Voldemort. Hermione learned about his death after the battle.
Snape hears Lupin admit this while hiding under the Invisibility Cloak and also hears Lupin explain that he was not part of Sirius's joke. Snape still thinks Lupin is helping Sirius, however, and attacks Lupin. He then lets slip that Lupin is a werewolf the next morning.
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.
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.
An enraged Molly Weasley engages Bellatrix in a duel and fires a curse that hits Bellatrix right over the heart, killing her.
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.
Harry describes Umbridge to Sirius Black, saying that she's vile enough to be a Death Eater, despite not being one. Umbridge is also depicted to be a prejudiced person, despising half-breeds and Muggle-born witches and wizards to the point of firing Hagrid due to his half-giant birth status.
George has his ear cursed off by Severus Snape (by accident), who uses the Sectumsempra curse. When he recovers, he seems in good spirits, making jokes about his new, earless appearance and by saying that at least now their mother will be able to tell them apart.