Monster infighting

In Doom, some of the enemy monsters can be tricked into fighting each other, or infighting. This can usually be accomplished by moving so that the player and two monsters are colinear. Since a monster in Doom does not have the intelligence to cease fire in the event that a fellow monster blocks his line of sight, said fellow monster will be subject to friendly fire. Monster infighting, as it is called, will soon follow.

According to Doom logic, however, monsters of the same type or species are immune to each other's projectiles (with the exception of firearms). This means that getting two of the same monster to fight each other is very difficult due to the fact that the above procedure will be ineffective. To cause homogeneous monsters to fight each other, the player must somehow get one of the monsters to melee attack and hit the other. This may be possible with the help of the invisibility power-up, which causes enemies to fire or attack the wrong direction. Monsters will also go after monsters that have caused them indirect damage by exploding a nukage barrel. In version 1.1 of Doom, a monster that had caused itself damage by destroying a nearby barrel would commit suicide.

Monster infighting has become a novel aspect of Doom and its sequels, add-ons, and spin-offs. Indeed, most games do not give the enemies the ability to hurt each other, much less the ability to retaliate. In more recent first-person shooters, however, enemies are intelligent enough not to fire if a fellow enemy is in the way.

Some very hard levels are not playable without this trick. There are so much monsters that the marine must make the monsters fight each others, in order to kill them all (because of low ammo) and even to survive.