Accidental Murder occurs when a situation that wasn't intended to be lethal ends with the death of someone anyway. Occasionally, this happens because a fight just goes too far (Bob and Alice start fighting, and the fight gets more heated than anyone expects, and Alice ends up braining Bob with a heavy object or accidentally knocking him onto something sharp... and she doesn't realize he's dead until it's over). Other times, this happens when innocent bystanders are caught up in someone else's fight and are dead before anyone else realizes they are there (Bob and Alice are fighting; Carol, a friend to both, tries to break it up and gets accidentally shot while Bob and Alice are wrestling for a gun). And sometimes there's no fighting involved at all; the person who dies is killed because the unintentional killer does something stupid.
Either way, it's simple bad luck for the dead guy, and for his killer. No one was meant to die, but someone died anyway. The ultimate result of an Accidental Murder varies. Bob killing Carol (or Alice killing Bob) may cause him to become The Atoner, or make a Deal with the Devil to bring her Back from the Dead. Often the character who committed the Accidental Murder will vow to become a pacifist, although it may just be the first step down a slippery slope of murder and mayhem.
See also I Didn't Mean to Kill Him, which is usually what Bob tells the police when he's arrested. Compare to Murder by Mistake, where Bob had every intention of killing Alice, but for some reason ended up killing Carol.
Legal note: By definition, in most legal systems "murder" requires that the death be intended by the killer, or the result of deliberately inflicted injuries. In most of the situations described above, the charge would be manslaughter or a fatal and unfortunate accident. However, that makes for a less evocative title.
As a Death Trope, all spoilers will be unmarked ahead. Beware.
Inverted: A bad guy manages to push Kenshin to the point where he reverts to Battousai and starts killing people, the first in line being said bad guy. Only it turns out that the sword Kenshin used was the other Reverse Blade Sword, meaning Kenshin has committed accidental non-murder.
Kenshin also accidentally kills his first wife, Yukishiro Tomoe, before the beginning of the series, which is what motivates him to give up killing in the first place.
Toboe unintentionally killed his elderly human owner/grandma.
Later he kills a girl's pet hawk.
In Mobile Fighter G Gundam, the rules specifically state that Thou Shalt Not Kill, but also that if a fighter is accidentally killed in battle, their opponent won't be held accountable. This plays into one episode's plot, where Sai Saici is hounded by a Back from the Dead fighter who died while fighting Sai's grandfather; at first he thinks the mummy wants vengeance, but it turns out that he just wants to have the fair fight that his death interrupted.
In Fairy Tail Zeref can do this by standing too close to people. Understandably, his arrival marks the end of the manga's days as a Nobody Can Die story..
In Higurashi: When They Cry during the manga arc Onisarashi-Hen (Demon Exposing Arc), it's implied that Aki Kimiyoshi's death, Natsumi's grandmother, actually was an accident due to Natsumi shoving her away and Aki hitting her head. However, Aki's death is still responsible for leading to Natsumi's Sanity Slippage, which Aki herself started to go through as a result of the Hinamizawa Disaster and fear of "Oyashiro-Sama's Curse." Natsumi blocked it out and hallucinated that her mother Haruko had stabbed Aki to death.
Early on in Bokurano, the cast is standing on top of Zearth after the first battle after the "tutorial", when Ushiro bumps Waku, and Waku seemingly falls to his death as a result, causing Ushiro to become angry when it seems as though everyone is accusing him of murder. It turns out later that Waku died as a result of piloting Zearth, and he was already dead before he fell.
In Perfection Is Overrated, Shizuru destroys Hitomi's Child while launching a surprise attack on her to save Mai and Natsuki, but since the Child is powered by the life of the person the Hime or SUE cares for most, and Hitomi cares for no one but herself, Hitomi dies in the process. Later on, Natsuki unintentionally kills an innocent bystander who happened to become important to a SUE, when she defeats that SUE.
In the Hunger Games fanfiction Some Semblance of Meaning, ValeWhitaker inadvertently kills the girl from District Seven, Cassia, when the latter hurts Vale's district partner and ally, Kit. Vale flies into Mama Bear mode, and before she can even realize what's happening, there's a knife in Cassia's heart. Vale feels very guilty about this later, though.
In Hood of Horror, Snoop Dogg's character, Devon, accidentally kills his sister, and sells his soul, becoming the Hound of Horror in exchange for her life.
In Walk Hard, Dewey Cox accidentally cuts his brother in half with a machete. Later, his father cuts himself in half.
In Pulp Fiction, Butch accidentally kills another boxer in the ring, though he shows no remorse for it.
In Daniel Deronda, Gwendolen hesitates just... one... moment too long when her Asshole Victim husband, Grandcourt, falls off their boat. He drowns, although she does make a belated effort to save him.
In Magnolia, a very convoluted, accidental murder is shown in one of the tales at the start of the film. A boy attempts to commit suicide by jumping from the roof of his building. His attempt would have failed as he would have been saved by the safety netting below, installed by window washers but... he is fatally shot with a shotgun, fired by his mother, as he passes their window on the way down. She was threatening the boy's father with the shotgun when it unexpectedly goes off. She later tells the police she that it was never kept loaded and that she and her husband often threaten each other with it all the time but... a friend of the boy's says that the boy, having his fill of his parents constant arguing, had loaded the gun the day before, hoping something would happen during the next argument.
That story was recounted in a Homicide: Life on the Streets episode, and is probably older than that.
Practical Magic has Sally putting belladonna into Jimmy Angelov's tequila to put him to sleep. Without something to put the safe amount in, she puts in too much and kills him.
In Tokyo Zombie, Mii-chan and Fujio's accidentally kill their boss by hitting him on the head with a fire extinguisher.
In Twelve and Holding, Jeff and Kenny accidentally kill Rudy while trying to burn his treehouse down.
In Very Bad Things the titular action kicking off a slew of death and destruction was the accidental death of a prostitute on a bathroom towel hook during coitus.
In the original Halloween II (1981) a speeding cop isn't able to brake in time and winds up hitting the drunken Ben Tramer, slamming him against a parked vehicle.
In Superstar, Mary Katherine Gallagher's parents were killed by their step dancing group stomping them to death accidentally during a performance, when the music was accidentally sped up.
Subverted in The Skulls, where Luke's best friend Will ends up dead, and he suspects his new friend Caleb. He watches the security tapes, and sees Will fall off a ledge to the floor below (which isn't enough to kill him), but Caleb grabs his legs to try to save him. He doesn't hold on, and Will falls on his head, and a cracking sound can be heard. Caleb assumes he accidentally broke Will's neck and leaves. However, Luke then continues watching and realizes Will is still alive, until Caleb's father's men arrive and finish off Will.
Partial example: In The Lion King, this is how Scar tries to convince Simba that Mufasa's death was his fault, and later tries to convince the rest of the pride of this. (Though it was actually Scar's fault.)
In Mouth to Mouth, Axe helps Manson (a much younger kid than everyone else) flip into a trash can to get food. Manson lands on a nail which hits an artery, killing him quickly.
Will Dormer accidentally shoots his partner Hap while they're tracking Kay's killer through the mist, but he covers it up because he had a motive for killing him. Towards the end however, not even he is sure whether it was an accident or not.
Subverted with Kay's death. Walter Finch claims that this is how Kay died, but he's just making excuses. When she died, rather than panicking, he meticulously and calmly prepared her body to remove any evidence. Dormer also reveals near the end that the research shows that he would have to have taken around 15 minutes to beat her to death, making it anything but accidental.
In Deep Rising, Canton accidentally axes Vivo in the head when he believes that it was the creature on the other side of the door.
In Tombstone, Curly Bill Brocius accidentally shoots and kills the town marshal, Fred White, while high on opium. Historically, the real life Brocius and White were friends, and everyone, even Wyatt Earp himself (who arrested Brocius after the shooting) said that it happened accidentally (and in the end, Brocius was acquitted of murder and the shooting ruled accidental). In the movie, this is made clear by Curly Bill repeatedly (and mournfully) asking, "Get up, Fred! C'mon now, get up!"
In the Dutch novel, film, and musical Ciske de Rat, the title character is only 11 years old when he accidentally kills his mother.
In Salamandastron, two weasels (who have taken refuge in Redwall Abbey) start playing with bows and arrows from a sports competition, until they accidentally shoot Father Hal in the neck, prompting them to flee out of fear of retribution. They end up accidentally framing the protagonist, who stumbles across the abandoned bows and is caught while picking them up.
In the backstory of Harry Potter and the Deathly Hallows, Albus Dumbledore, Grindelwald, and Aberforth (or any combination thereof) accidentally kill Dumbledore's sister during a duel.
Heralds of Valdemar: In one of the short stories in Oathblood, Tarma accidentally kills the lord of Viden when he attacks her because he's drunk and he tripped over Worrl's tail.
In The Drowning of Stephen Jones, the murder of Stephen Jones is CHARGED as accidental manslaughter. It wasn't, but hey, the judge said so.
Done by Parrish in Parrish Plessis. When Parrish attempts to rescue the kidnapped Wombebe, she ends up accidentally killing the poor kid herself with an explosion that was intended as a diversion.
In Billy Budd, the title character accidentally strikes the villain to death. The bastard deserved it though.
In Darkness Visible, Marsh opens an unstable Threshold for him and Lewis to escape through, and slams it shut after them. Unfortunately, their pursuer was already half-way through at the time.
At the end of Ender's Game, it is revealed that Ender had previously killed the school bully he fought in kindergarten, and later Bonzo in Battle School. Although, it is strongly implied that, at some level, it may not have been fully intentional.
At the end of Speaker for the Dead, it is revealed that the Piggies killed the xenobiologists because they thought they were preparing the biologists for the next stage in their life cycle, like the Piggies.
In the first book of The Hunger Games, Peeta goes to collect some berries. However, his partner points out they are poisonous. A few moments later, Foxface (another contestant in the Hunger Games) dies after stealing some supplies from the duo after seeing Peeta pick the berries. Needless to say, Peeta is pretty ashamed of picking the berries in the first place.
In the third book Peeta, while driven insane from hijacking, attacks Katniss. Another member of the Star Squad tries to pull him off her and Peeta throws him aside, accidentally sending him right into a barbed wire pod, killing him.
Happens in the Lord Darcy case, "The Ipswitch Phial".
In the fifth Warrior Cats arc, Dawn of the Clans, the protagonist Gray Wing is out on the moor minding his own business when his brother's henchcat Fox attacks him. In the ensuing fight, Gray Wing accidentally kills Fox by slashing his throat. This leads Gray Wing's brother Clear Sky to declare their brotherhood over, with his hypocrisy reinforcing how far he's fallen.
Heroes: Isaac and Peter are fighting, intending to kill each other, then Simone walks in and get shot by accident.
Also, when Sylar/Gabriel is attempting to calm down his mother, he accidentally stabs her in the heart with the scissors she was wielding.
An episode of CSI: Crime Scene Investigation involves the murder of a famous poker player known as the Candyman. Several people want to get back at him for something, but none claim to kill him. One drink server admits to putting eyedrops in his drink, but only to give him diarrhea. The connection is that the Candyman's favorite table munchies were a certain type of chocolate piece made from chocolate from West Africa, where they still use leaded gasoline. Thus, there was enough lead in his system to interact with the eyedrops and spiked his blood pressure enough to kill him; result, literal death by chocolate.
Another episode deals with a college student who goes missing the night she is supposed to return home. It turns out she was cleaning out her dorm room, went to empty her trash, and accidentally dropped the trashcan down the chute. Knowing she wouldn't get her security deposit back if the trashcan was missing, she tried to retrieve it from the outside Dumpster. Unfortunately for her, a passing motorist just so happened to accidentally ram the Dumpster at the precise moment she had climbed up the back and was leaning into it, pinning her between it and a back wall and crushing her. She later fell into the Dumpster where she bled to death. Her body was fed unnoticed into the trash compactor.
Her parents refuse to listen to this explanation, convinced that she was murdered by someone. It's human nature to want to blame something (or someone) specific instead of a series of unlikely events. Grissom is actually confused by this, as facts are everything to him.
And then we have the guy who slipped on Jello and in the process accidentally stabbed his wife with a shard of glass.
Same episode, same guy. One of his neighbors saw him kill his wife. When he tried to stop her from calling the police, he accidentally ran her head-first into her phone, killing her as well.
Same episode, same guy. During his childhood, he accidentally cut off the oxygen to his grandmother's nasal cannula (nose-tube thingy), causing her to suffocate.
Also the Season Two episode where two men got into a heated argument over the sale of their shared-ownership land, and the first punched the second in the chest —unwittingly causing a cardiac arrhythmia and stopping his heart.
Then there was the Firing in the Air a Lot case where a stray bullet shot accidentally into the air (instead of a target in the shooter's backyard) ended up killing someone halfway across town.
In early seasons, especially, these types of accidental slayings were pretty common. Especially when perpetrated by a guileless party, like a child or someone just going about his business. Sometimes it would cause the investigators to angst over the necessity of upholding the law despite their sympathy for the "murderer."
A shootout in a mini-mart results in all the crooks dead. Unfortunately, an innocent bystander is also shot during the firefight. It turns out that the cop was aiming for the crook who was running and shot at him at the exact moment he was passing the bystander. The bullet hit the crook, came out the other side, and killed the woman. The cop got a commendation for his actions.
And then there's the worst case of a Type 2; Warren fires wildly into the air after having shot Buffy, hitting Tara through the window and driving Willow insane with grief.
In LOST, after hearing whispers in the forest, Ana Lucia accidentally shoots Shannon. Later on, while Michael kills Ana in cold blood to let Benjamin Linus escape, Libby walks in and he shoots her in a panic.
On Castle, it turned out the murder victim was having an argument with a client that was angry, but not bloodthirsty, and the killer pushed her backwards — not realizing she would fall and be impaled on the spikes of a tiny garden.
In Misfits the protagonists have ended up killing multiple people, and would have killed more if Curtis hadn't rewound time to prevent them killing their third probation worker.
In Bones, a man is killed while having an argument with a colleague, who pushes him, not realizing that there is a plane propeller spinning up behind the former. This would not be a murder, except the accidental killer and his wife then decide to cover it up by dropping the body from a plane.
In The Practice, Bobby hires a hitman to intimidate exonerated (but guilty) serial killer William Hinks into ceasing his harassment of Lindsay, but the hitman kills Hinks instead. Bobby is legally responsible for Hinks' death regardless of his instructions to the hitman, but is acquitted anyway.
The explanation for a good chunk of the murders on Cold Case. The final flashback often reveals that the killer accidentally shoved the victim to his or her death in the course of an argument or struggle.
Once Upon a Time: Red accidentally kills her mother when the latter tries to kill Snow.
Orphan Black: Beth was suspended from active duty because of a line of duty shooting of a civilian. Though it turns out it wasn't accidental. Or a civilian.
Aishiteru: What happened between Kiyotaka and Tomoya was a children's fight gone horribly out of hand. It led to Kiyotaka's death.
Are You Afraid of the Dark?: In "The Tale of the Night Nurse", a girl begins to experience the death of a young woman many years ago at the hands of a murderous nurse. However, it turned out the dead girl was allergic to penicillin and wore a medic alert bracelet that fell off by accident. When the girl finds the bracelet and shows it to the nurse, the nurse angrily chides her for taking it off, saying she might have killed her. This changes time and the dead girl goes on to live a full life.
In the Irish ballad "Molly Ban", Molly's boyfriend mistakes her for an animal while hunting.
Romeo and Juliet - Tybalt kills Mercutio, bringing a plague on both Montagues and Capulets. Though it's up to the show whether their fight was in deadly earnest, or if they were just fooling around and it got out of hand. What everyone agrees on is that Romeo really messed it up.
Persona 3: Shinjiro learned the hard way about losing control of your powers. When chasing a Shadow, he lost control and ended up killing an innocent woman. Who left behind a vengeful son. Tragedy ensues.
Persona 4: The first murder was a complete accident. The culprit was trying to rape the victim when he accidentally pushed her into the TV. Averted in the case of the members of the Investigation Team, since the second fall guy note The first being a kid who commits an actual murder and tries to make it look like one of the serial killings so he can claim credit for all of them. honestly believed that pushing them into the television world would protect them from the real killer (because of his manipulations). Obviously the trope is still played straight when the player fails to rescue a trapped person in time.
In Mafia II Joe Barbaro gets drunk after his friend is killed and starts waving his gun in the bar. When situation appears under control, Joe suddenly slams angrily his gun against the table accidentally pulling the trigger and killing the barman.
At the climax of Silent Hill: Homecoming, Alex finally remembers that Josh has been Dead All Along, and his own role in it. During a fishing trip in a nearby lake, Alex, finally fed up with how his parents had always neglected him in favor of Josh, gets into an argument with Josh over their family's signet ring. During the argument, Alex accidentally knocks Josh off the boat, breaking his neck in the process. The final boss, Amnion, is essentially a manifestation of both Alex's guilt over Josh's death, and Josh's desire for Alex to accept responsibility for his role in it.
In the CSI Nintendo DS game: One case involves the team investigating the death if a man who apparently got impaled on the sword of a statute. It turns out that a man who was angry at the victim stabbed him with a knife with the intent to harm but not kill. The victim in, drank some wine to calm himself which caused his body to go numb. While walking around on a balcony overlooking the statue, the victim, due to this injury and the state of his body caused by the injury, falls over the side and is impaled on the sword of the statue. The man who stabbed the victim gets arrested for causing the man's death through his actions.
Super Mario Bros. The instruction booklet for the original 1985 game specifically states that "the quiet, peace-loving Mushroom People were turned into mere stones, bricks and even field horsehair plants..." Every brick broken is a dead citizen, every fire flower Mario eats is a dead citizen and accidental cannibalism. Every struck coin block is a dead citizen. Mario Mario and Luigi Mario: accidental mass murderers.
The "Bad Boys Love" route of Hatoful Boyfriend is kicked off when the heroine is found dismembered. Much horror-mystery and exposition later, it is found that Dr. Shuu was secretly weakening Ryouta Kawara's body to make him the carrier of a fast-acting lethal-to-humans virus for use as a bioweapon, and the (human) heroine died when she went to the infirmary to check on her friend Ryouta and was exposed to the virus. This revelation sends Ryouta across the Despair Event Horizon.
Twisted in Super Dangan Ronpa 2: the murder is accidental on the part of the murderer... but not on the part of victim. Komaeda deliberately arranges for him to be killed by the traitor in the hopes that the rest of students would convict the wrong killer, which will result in everyone but the murderer being executed. He is indeed killed by the traitor, Chiaki Nanami, but once she realizes this, she has everyone else convict her so that she's the only one executed.
Gaige from Borderlands 2 created her robot, Deathtrap, as a slightly overpowered bully deterrent. When her classmate Marcy pushed her at the science fair, Deathtrap registered Marcy as a threat and Marcy, well... exploded.
In Kiss Of Revenge, the protagonist's mother died in surgery twelve years ago due to mistake on the part of the operating surgeon. This sets off the game's whole plot, as the protagonist spent the following twelve years planning revenge for her mother's death and how it was covered up.
In Schlock Mercenary, a set of mines set for security purposes accidentally blow away King LOTA, resulting in the second page quote ("you've just invented 'negligent regicide'"). However, like every good robot dictator, LOTA had a secure backup system.
Banished: Rak is held captive on Strix 13 due to this crime.
Homestuck: A cultural misunderstanding regarding human childcare/safety turns out to be responsible for the death of Jade's grandfather in the backstory. Tavros is, uh, very sorry, about that.
Played with in EerieCuties. A demon that came out of a magic mirror in Nina's form tied vampire hunter Tiffany's shoelaces together to trip her. Unfortunately, Tiffany was carrying a sword and impaled herself on it when she fell, becoming a ghost, temporarily. However, it turns out that she's not really dead, the sword causes someone's astral form to be thrust from their body, while leaving the body intact. One vampire bite later, Tiffany is back where she belongs, albeit with other concerns now.
This happens quite often in Survival of the Fittest. A notable example during V4 is Kris Hartmann getting spooked by Reika Ishida and shooting her in the chest. Eric Lorenz's death at the hands of Alex Rasputin was also accidental.
In V3, Abel Williams is accidentally tripped up by Michael Anders and falls over... hitting his head on a rock and dying.
In Sex House, the housemates accidentally kill the repairman they took hostage by keeping him in a room full of mold.
In The Boondocks, Granddad accidentally kills Colonel H. Stinkmeaner in the rematch to their fight. While everyone assumed Stinkmeaner had a Disability Superpower, he turned out to just be a blind old man who got lucky.
In The Prince of Egypt, Moses tries to stop an Egyptian from beating a slave and (possibly) accidentally shoves him off a multi-storied scaffold.
This is Bowdlerizing the original story somewhat: in the biblical account, Moses deliberately killed the Egyptian in question.
In an episode of Jonny Quest: The Real Adventures, a man and a woman were arguing and the man accidentally pushed the woman down the stairs and killed her (broken neck). How did he cover it? By burying her in a wall.
This happens to Kenny in South Park where Stan and Wendy uncover Bebe's deep-lying conspiracy to get free shoes. Wendy and Bebe end up fighting for a gun, and after several seconds, a shot fires out. Wendy and Bebe both back away... but neither is injured. We then cut to Kenny eating dinner with his family, when the bullet manages to hit him.