Kill Steal

Freddy Krueger: "She's mine! MINE! MIIIIINE!"

—Freddy vs. Jason

Simply put, someone is about to kill a person, and someone else leaps in and does it instead, usually to get the credit and/or reward.

Say Bob the Avenger is locked in mortal combat with the warlord Baron Von Bloodlust. Then the Baron gets knocked down. Bob has him. He brings up his sword to make the final strike... and then the Baron falls down without Bob touching him. There's a knife in his back, held by Alice Dreamrider. She just smirks, and says "Too slow," before swaggering away.

What the hell? Sure the Big Bad is gone, and there is peace to the world, but dammit, that was just dishonorable!

This is a common way to establish a rivalry in stories about killers. Stealing a kill is a sure way to piss these characters off. Heck Body Count Competitions often encourage this.

Happens with players in Tabletop Games (especially with The Munchkin) or online games (especially with Griefers). There is a significant difference between it happening in an FPS and an MMORPG though. In an FPS things tend to be much more fast paced and the consequences of losing a kill aren't much more than the loss of one point (and maybe a few wasted bullets) so complaining about it too much is considered quite childish. In an MMORPG things are slower paced so it's always clearer who's fighting what (so a KS is more likely to be deliberate) and a single kill usually involves more effort and grants a lot more resources (in the form of loot and XP), as a result Kill Stealing is a lot more of an issue. This is best summed up in the solutions to the problem:

In an FPS you can get points for "assisting" in a kill.
In an MMORPG you can't attack someone else's target without their permission (or alternatively if you attack something already fighting someone else you won't get anything for it).

In general, kill stealing in video games will refer to a combat oriented online game, and the actual act is killing an enemy who's heath has been whittled down by another player but then one person comes along when they are almost dead and kill them despite not having dealt any damage before. It is often seen as a dishonorable act by other players. There are a few cases where it is helpful, such as an MMORPG where a lower level player can do this on a higher level monster to get more XP.

Compare The Only One Allowed to Defeat You, Ninja Looting.

Contrast Helping Would Be Killstealing.

Examples of Kill Steal include:

Anime and Manga

Isamu's does this to at least one other pilot in the opening sequence of Macross Plus. This is one of several bad habits that eventually get him re-assigned as a test pilot for Project Supernova.
In Tiger and Bunny, superhero Pao-Lin Huang/Dragon Kid has a regular habit of stealing other heroes' arrests (and thus their points and hero ranking) at the last minute.
Subverted in Berserk. At first glance it looks like the Skull Knight's appearance during the final battle with Ganishka qualifies, but the dialogue reveals that his stroke was actually meant to hit Femto, who was about to fight Ganishka. It's debatable whether Femto actually planned this.
Chloe's first appearance in Noir had her killing two targets that the main characters had accepted a contract on. In a later episode, Chloe then goes on to protect her target from a group of assassins sent by a third party to prevent this trope.
In the later seasons of Pretty Cure once the cures learn an attack that uses the entire team you can bet it'll be used against every monster even when one of the Cures is clearly capable of defeating it by herself. Notable:
- Smile Pretty Cure has an entire arc wherein each episode can be summarized thus: 1) Cure X fights Monster of the Week by herself. 2) Cure X gains a new attack and uses it against MotW 3) MotW barely survives 4) other cures show up to finish off MotW.
- Happiness Charge Pretty Cure has the fight between Cure Lovely and Seiji take place on another planet because Lovely wanted it to be a one on one fight, so naturally the other cures so up to "help" at the last minute.

Film

This is what sets off the "versus" in Freddy vs. Jason, as seen in the quote. Freddy resurrects Jason and sends him to Elm Street, hoping that Jason's rampage will provide him with a fresh source of fear. Everything goes as planned right up until Jason stabs one of Freddy's sleeping victims before he could kill her himself.
Not directly stealing, but Alice doing this to Jill a few points in Resident Evil: Apocalypse is a major reason she's considered a Canon Sue. See Phelous's reaction to them here.
- The unfortunate trend continues into the sequel, Resident Evil: Afterlife.

Live-Action TV

In an episode of Highlander, Amanda (who is, after all, a thief) takes the head of an Immortal antagonist whom Duncan MacLeod was just about to finish off. As a result, she gets the energy from the Quickening.
- Methos does this after Duncan refuses to kill a female immortal, citing "It had to be done"
- Part of the reason that Immortals in the Highlander verse insist on duels instead of ganging up is to minimize the chance of someone stealing a kill and gaining the energy from the defeated Immortal's Quickening.

Tabletop Games

A main theme of the game Cutthroat Caverns. Players represent a party of adventurers who must work together to fight and defeat various monsters...but only the player who delivers the killing blow gets points. The game's tagline: "Without teamwork, you will never survive. Without betrayal, you'll never win."

Video Games

This is a popular tactic in Halo online multiplayer, let someone else weaken a foe, and then swoop in and steal it.
- In the single-player game, the A.I. controlled Marines will complain about this if you kill the enemy they're targeting; even if you're the one who actually landed the majority of hits on it.
- Halo: Reach actually has a specific medal just for stealing an assassination by killing the person being assassinated before the animation ends.
  - It's called "Yoink!" and it's used as the Swear Filter on 343 Industries' forums. For the inverse, killing the assassin (if he's a foe), you get Showstopper.
Henry does this in a cutscene for one of the bosses in No More Heroes.

Travis: Where are your manners? That was my kill you naughty boy!

In the Super Smash Bros. series, when someone gets knocked off the stage, the KO goes to the last character to land an attack on them. This means that if you're quick enough, you can get credit for the KO even if another character is the one who does them in.
In Star FOX, Fox's wingmen will complain if he shoots down an enemy they were taking care of.
In Final Fantasy VIII Seifer Almasy invokes this trope during the tutorial. He orders his comrade (you) to leave him the last hit on all monsters, because this way he gets more XP. Classy.
In Fable II, if you wait too long to kill Lucien, Reaver will shoot him instead.
This is used to train weaker soldiers in Fire Emblem games. Have your stronger fighters weaken the enemy then have the unit you're trying to train score the finishing blow. All other things being equal, the unit that inflicted the killing blow will gain more experience than the other fighters. This combined with the fact that units gain more experience if they have a lower level than the enemy means that your weaker units can soon become bonifide powerhouses.
In Flying Heroes, you may order your AI companion to steal kills. As it is a four-team deathmatch, the companion will take kills from the other teams rather than yourself.
Vindictus averts this. No matter who killed the monster, everyone gets a share in the XP and loot. It does open the possibility of party leeches, but they can just be kicked from the party.
In Phantom Brave, Bounty hunters are called "Chroma," and the man named Walnut is a "Chroma Oxide," someone who specializes in ripping off the bounty other Chroma are getting, either by this trope or by defeating the other hunters directly.
Ragnarok Online allows you to configure equal XP share or damage proportional XP share for all party members. Loot is still dropped on the ground.
Gears of War has players go into a "Down But Not Out" state when they take enough damage. In this state they can be revived by a teammate or killed by an enemy. The frequency of players killing the downs of other players is perhaps one of the major factors that led to the series' severe Complacent Gaming Syndrome regarding the shotgun, which, in addition to being just better, could blow an enemy to Ludicrous Gibs without the need of a down, and if you did get a down with it you were close enough to claim your kill quickly before anyone else could.
League of Legends delivers bonus gold to the person who lands the killing blow, although anyone who's helped recently gets some. But more importantly, how many kills you get is recorded on your match statistics. Naturally, the arguments are bitter. Not helping is the extremely large amount of factors, many of which are unlikely to be discernible to someone intervening, that make the difference between someone dying and someone managing to limp back to safety with barely any health left.
A viable tactic in Dokapon Kingdom (and its portable counterpart, Dokapon Journey), where you can interrupt a fight between another player and a monster and deliver the killing blow to take all the credit for yourself.
The World of Warcraft quest chain that has you decide between two factions in Sholazar Basin begins with a Wolvar accusing you of this when you kill a jungle cat for a completely different quest.
- In the final battle against Deathwing, the players nearly win on their own when Thrall shows up just in time to get the glory of the killing blow.
There is actually a achievement in Red Dead Redemption multiplayer to allow someone to steal your kills 10 times in a gang hideout.
Although City of Heroes initially played this trope straight (leading to more than one conflict between solo players and those sincerely roleplaying helpful heroes), the experience award system was eventually revised to distribute XP among all those who had damaged a target in roughly the proportion to the amount of damage they had done to it. (Unless, of course, they are all on a team together, in which case everyone gets full XP.) This meant that someone who slides in at the last minute and cherry taps a villain with a couple HP left gets only a tiny percentage of the XP for it.

Web Comics

The heroes of DM of the Rings complain when Gandalf does this.
The Order of the Stick: Sabine doesn't like this either.
- Nor does Xykon.
- Belkar has done a lot of vile things, but he draws the line at kill stealing. Well, he does now.

Web Original

Spoony's Campaign: "We were fighting like men! We were locked in honorable combat, and then you came in and fucked it all up!"

Real Life

One of the most famous property law cases, Pierson v. Post, involved Pierson shooting and killing a fox that Post was pursuing. Post sued for the value of the fox, claiming a right to it by virtue of pursuit. The court held that while kill-stealing might be rude, a person doesn't get any property right in an animal just by chasing it.