The line comes from the Halo first-person shooter series, where the multiplayer announcer calls out Betrayal when a player kills someone on their own team and earns the related penalty. Voiced as part of the standard match announcements alongside medals and game events, it became familiar to players through years of local and online multiplayer. Clips and montages later carried it beyond the game, and creators now borrow the announcement as a shorthand label for team kills, backstabs and broken alliances in other games and in real-life skits.