In online games and especially first-person shooter games, MMORPGs and MUDs, kill stealing is the practice of arranging to get credit for killing an enemy, when it should have clearly been another player's kill. An example is when a player in a first-person shooter whittles an enemy's health down and is about to kill them, when another player comes along and shoots that enemy once. The second player gets credit for the kill despite having done almost none of the work of killing that enemy. Kill stealing occurs when the rewards for defeating a foe are limited or highly desired, and many players are competing for that same reward. The term is most often heard in MMORPGs and MUDs, where rewards of items and experience points can be substantial, but can also be found in first-person shooters where players are rewarded a point for a kill. Many players feel that kill stealing is a dishonorable practice.
There are two main causes for kill stealing: the desire for the reward and the desire to cause other players grief. Kill stealing is predominantly done to gain the rewards from a kill. Griefers kill steal as only one of their tactics in annoying other players.
One of the most prominent games where the term kill stealing is heard is the fantasy MMORPG EverQuest. In this game, the experience points and right to the rewards for a kill goes entirely to the player or party that did the most damage to the defeated mob. Some player classes were designed to do more damage than others and could find it quite tempting to "help" other players kill monsters. For example, a warrior might be battling an orc only to have a spellcaster come along, blast the orc to death in an instant and get all the reward, while the warrior walks away from the fight, having lost time and hit points, and gained nothing. This current system is still considered to be an improvement over the original method of awarding experience and right to the rewards that the game began with. In that system, the determining factor was which player did the most damage. If that player was in a group, then that group got the experience. However, if one person outside of the group did more damage than any single player in the group, the experience went to that outside person, even if their damage was far less than the combined damage output of the party. Widespread killstealing of entire groups by single players led Sony to implement the currently used method, which considers the combined damage output of the entire party.
Complaints of kill stealing are sometimes heard in online first-person shooters. In most of these games, the credit for a kill goes to the player who deals the killing shot. Players usually ignore complaints of kill stealing in FPSs because the rewards are less significant and because these games move much faster (i.e., it was probably accidental). Furthermore, in FPS combat, players are usually either allies (in which case the kill's credit going to one player or another has no in-game meaning beyond ego) or in direct enmity with one another (providing both a justification for cutthroat tactics, and a generally immediate means of redress). Kill stealing can sometimes specifically refer to the disruption of a particularly "interesting" kill that would have earned a lot of extra points beyond the kill itself. This type of kill steal is sometimes referred to as a "yoink."
By contrast, in most MMORPGs, players may be competing for the same in-game resources, but are not generally in direct conflict with one another. (In situations where they are, such as two opposite-faction players in a World of Warcraft player-versus-player server, there is usually little animosity towards kill-stealing, as there is a means of redress and prevention, and it is seen as part of the general struggle between Horde and Alliance)
Kill stealing is a rare event in real-time strategy games (for similar reasons as with FPS games, but also due to the larger amounts of killable entities), but does however appear in some forms. In the Defense of the Ancients map in Warcraft III, kill-stealing can frequently happen due to the money reward given to the player that delivers the killing blow to an enemy hero or creep (although kill stealing creeps is not considered a big event, as the kill would often go to the AI and thus give no money reward to any player). There is also a particular version of kill-stealing called denying, which comes from the fact that friendly fire is near non-existing (harmful spells generally must be targeted at enemies, and regular attacks can only be performed on allies with a low amount of hit points). Because of this, there is no additional penalty for killing someone from your team (as in many FPS games) beyond the penalties suffered by the player killed. Players will therefore attempt to kill a friendly player or tower who is deemed impossible to save, thereby denying (hence the name) the enemy team the gold and experience rewards for the kill. It is also possible for certain heroes to commit suicide (through abilities costing them hit points, or which kill them outright, or by running into "neutral creeps" with no team affiliation in such a manner that these kill the player). Suicide is the most frequent form of hero denial, while creeps and towers (as they are impossible to control) are generally denied by attacks from a friendly hero. As denial only hurts the enemy and is harder to pull off, it is considered skill rather than griefing.
Kill stealing is usually not heard in situations where a cooperative goal is being sought. One member of an MMORPG hunting party will not argue that another member of their party is "kill stealing" from them. Any help towards the cooperative goal is desired with the understanding that the rewards will be shared fairly amongst the participants. The focus of some classes on damage dealing, others on withstanding punishment, and still others on healing or assisting the others even encourages this activity. However, in situations where individual goals are sought concurrently with cooperative goals, as in the online artillery game Gunbound, accusations of kill stealing and requests to refrain from kill stealing are common.
Preventing kill stealing 
Many newer MMORPGs implement game designs that distributes the reward more fairly to those who fought a creature. Rewards can be distributed based on how much the player contributed to defeating the creature. A player that does 30% of the damage gains 30% of the money and experience points rewarded for defeating the creature (as it is in City of Heroes). A game might have a more sophisticated way to measure a player's contribution to the fight as well. A character whose primary task is healing other characters might be judged based on how much he healed combatants during the fight. In theory, activity based skill progression, rather than "experience point" based progression would also remove the non-drop based incentives for kill stealing, but this is, as yet, not a mainstream solution.
EverQuest 2 has a system of locked encounters that prevents players from helping others in killing a creature unless help is requested. Once the warrior attacks the orc, the wizard cannot attack the warrior's orc unless the warrior uses a special yell command for help. Tales of Eternia Online and Final Fantasy XI both use similar systems.
Guild Wars has a system where the item rewards from kills are randomly assigned to a player and the gold is shared among the party. This means that also a supporting character such as a monk or ritualist will get their fair share of loot, even if they didn't damage the enemy at all. Guild Wars 2, in an attempt to improve social cohesion among players, has a system where all players involved in a kill will receive loot. The loot (which includes gold) for all players however, will be different.
Flyff has a system where if one player has targeted a monster, no other player can attack that monster unless they are in the first player's party.
In World of Warcraft solo play, the player who "taps" an NPC by attacking or using an offensive ability on it gets experience points for the kill and the ability to loot it, NPCs remain tapped until they leave combat. NPCs under the player's control — called minions or pets — do not fulfill this requirement. The monster is "greyed out" to other players. If the monster is skinnable, no other player can skin it until the original player loots it, but there is nothing, so far, to prevent another player from camping the corpse and stealing the skin. In group play, each living party member near the monster on its death gets experience and kill credit, as well as loot. The default loot setting is round robin starting with the party leader, but rare items are rolled for by a percentile die (d100). In the loot system, some items are freely tradable ("BoE," or "Bind on Equip"), whereas others cannot be traded to other players, and can only be used or sold to NPCs ("BoP" or "Bind on Pickup"). It is considered bad form to roll on a BoP item without clearing it with the group first, or to roll on an item that your character cannot use when another character has expressed a need for it, akin to ninja looting.
In Disney's Toontown Online, if the player has already attacked a cog and another player joins in and finishes the cog off, the first player still gets the experience points for the damage done to the cog, effectively ensuring that players that partake in the combat get their share of experience points.
In Dystopia, players are given points for kills or assists. At least one point is given to the player who deals the fatal blow, but other points are distributed according to damage, ensuring that a kill stealer will only get a limited amount of reward.
In RuneScape there are single-combat areas where only one player may attack an NPC or a monster at a time, which makes kill stealing impossible. There are also multi-combat areas where any number of players can attack an NPC together, in which case the player that inflicts the most damage sees the loot and can take it; after 30 seconds all players can see it. Players working as a team to defeat powerful monsters generally use LootShare, which shares the loot from killing monsters among all members of the team, and automatically tells everyone else on the team what the loot was, and who actually received it. Which team member gets the loot from a particular kill is somewhat random, but it is also based on that player's previous loot, and that of his teammates. If a player has received valuable loot in the past he becomes less likely to receive the loot from a particular monster kill when using Lootshare.
In Perfect World International experience is distributed according to one's contribution so that a player who merely runs in and lands only the killing blow gets relatively little EXP, as opposed to if they had taken on the target all by themselves and won. However for quest purposes kill is counted to whoever touched the mob first and also the loot is protected for some time in their favor. For example, weak Blademaster started killing some beast and a Wizard comes and does the most of the damage - most of the EXP goes to a Wizard, but a Blademaster still gets his kill and also can take the loot first. Also, while one may enter a dungeon regardless of whether or not they have the relevant quest (referred to as the infamous "fb"), those who do so for their quest receive a special quest item that will let them enter an instance of the dungeon exclusive to the party ("squad"), and the holder of the quest item will upon successful completion of the quest be guaranteed to receive their quest reward(s) along with certain future quests and interactions.
In Mabinogi players are allowed to kill a monster another player was previously attacking. If the player who did the most damage did not kill the monster, the loot is not dropped and the highest damage dealer is given the chance to 'finish' the enemy, gaining full experience and dropping the loot that it had. The loot is soulbound and cannot be picked up by other characters.
Many competitive shooting games divide points for killing enemies. In some cases such as Call of Duty 4: Modern Warfare, players who deal damage to enemies but fail to complete a kill are given a static amount of points regardless of the amount of damage done. In other cases, such as Call of Duty: World at War and Gears of War 2 the amount of points awarded to assisting players corresponds to damage dealt to enemies. There is further variation to these systems, shown in Call of Duty: World at War rewarding players fully for kills and awarding bonus points to assisting players while Gears of War 2 splits points proportionally by damage (although a guaranteed 10 points out of 175 are given to the killer) and thereby ensuring that the player who got credit for an assisted kill does not receive as many points for such a kill as an unassisted one. Further deviations of such systems exist in other games of the competitive shooting genre.
See also 
- Camping — another practice that arises when players compete for rewards
- Ninja looting — improperly taking the reward from a defeated creature
- Powerleveling — powerlevelers are frequently blamed for kill stealing
- Team killing — player who intentionally attacks or kills his own teammate in multiplayer computer or video games
- Gold farming
- Peter Kollock (2002), Communities in Cyberspace, Routledge, ISBN 978-0-203-19495-9
- Jennings, Scott; Macris, Alexander (2005-12-19). Massively Multiplayer Games For Dummies. For Dummies. p. 310. ISBN 0-471-75273-8.
- Shah, Rawn; Romine, James (1995). Playing MUDs on the Internet. John Wiley & Sons, Inc. p. 130. ISBN 0-471-11633-5. "Under no circumstances let someone steal your kills. If the Gods don't deal with it for you, you have every right to do it yourself."
- "Kill Stealing: A Logical Deduction",[dead link] an opinion article on kill stealing in City of Heroes and MMORPGs in general.