Overview
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) 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.
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