Minecraft wiki entry for Monster_infighting, associated text: Once a mob has taken damage from a mob or player, that mob or player is set as the mob's permanent target. This means that even if another mob or player attacks an infighting mob, the infighting mob does not stop trying to attack its permanent target. If a certain mob refuses to infight with another monster, it is likely because the player or a mob already attacked it, so the player or a mob was set as the mob's permanent target. That mob does not infight with any other mob until the player or mob dies or goes out of tracking range. This mechanic can be useful in Survival multiplayer. One player can attack a mob and run away from it while another player kills it. Because a player or a mob already attacked the mob, it never retaliates against another player or mob.