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In Halo multiplayer terminology, a betrayal occurs when a player kills one of their own teammates during a team match. The act of deliberately betraying one or more teammates is referred to as team-killing. In Halo Campaign terminology, a betrayal occurs when the player kills too many of their NPC allies.
In the Halo games, in order for the betrayal to occur, the player must be completely responsible for killing allies. When killing one ally, others will be shocked but will not fire on the player. If the player continues to keep killing their allies, all allies in the area will start attacking the player until the player dies. Basic NPCs like Marines deal standard damage, but special allies like Sergeant Johnson, the Arbiter, Mickey or Dutch will be able to instantly kill the player regardless of weapons. If Campaign Scoring is turned on, killing allies results in a loss of points, which varies depending on how the betrayal occurred. Refraining from killing allies for an amount of time will cause the player's NPC allies to forgive and stop attacking.
In order for a betrayal to occur, the betraying player must be completely responsible for the death of his or her teammate. Enemy players must play no role in the act. For example, if two players on opposing teams are battling each other, and a third player accidentally kills his already-damaged teammate with a grenade, it will not be counted as a betrayal and the enemy will be credited with the kill. On the other hand, gunning down a teammate while no enemies have damaged him counts as a betrayal.
In Halo 3, "betrayal booting" was introduced. Players who commit too many betrayals can be booted by their most recent victim. The booted player also receives the penalty for not finishing the match. In most Matchmaking gametypes, the boot threshold is set to two betrayals. Halo: Reach and Halo 4 work differently:
- “1 betrayal isn't enough, on its own, to prompt your teammate to boot you.
Every player has a value that tracks the damage they've done to their teammates, and the system that governs that value takes into account many things. The value has a threshold, and if you commit a betrayal that pushes you over the threshold or if you are over the threshold and commit a betrayal your teammate will be prompted to boot you. The value is in a constant state of degradation but it decays slowly and carries over between games. This is done to ensure that a problematic player doesn't have their slate wiped clean and regain the ability to commit excessive flagrant actions against their allies every time they load into a game. A player may have two or three betrayals in a game and not get booted, but when that's the case there is a good chance a teammate in the next game will be prompted to boot them on their first betrayal in that new game.”
In Halo 5: Guardians, betrayal booting is not present. Instead, the game will automatically boot the player once they have betrayed three times, be it intentionally or unintentionally, and display a rather humorous message:
Et tu, brute?
You were returned to the lobby for betraying a teammate.
You wanted to triple-cross them, really.
Note that being booted from the game like this is considered by the game as quitting a match.