Have you noticed how, in video games, especially the platform kind, enemies don't kill each other? By that I don't mean that they somehow are so clever that they manage to target the player, and avoid the other enemies. No, if they collide, they just bounce back, and if they shoot or throw any flames, only the player gets hurt by it if hit. If any other enemies get in the way, the bullet or flames just pass right through them.

Static HP-tapping obstacles such as spikes or pools of acid act the same way. The player might very well die instantly at the touch of such objects, but I have never in my entire carreer as a console player seen a monster die or even take damage after falling off an edge and onto a floor of spikes.

I can think of at least two plausible explanations for this behaviour:

  • Calculating damage, changing sprites to reflect damage, removing sprites etc. is more CPU- and memory-intensive than mere collision detection. This is not so much an issue with today's powerful systems, but game genre conventions tend to linger for a long time, and the platform genre was started on very slow, weak computers.
  • A naïve implementation of friendly fire among enemies could easily cause most of the enemies to kill each other off, thereby making levels much easier to complete than intended. Programmers would have to extend the enemies' AI to keep them from killing each other, thereby putting a lot of efforth into realism, with little payoff in improved gameplay.

A limited kind of friendly fire does however exist in the game Chip 'n Dale: Rescue Rangers for the NES. Amusingly enough, this only happens between players. C&D RR has a co-operative two-player mode, and if an object thrown by one squirrel hits the other squirrel, the other squirrel will see stars and be immobilized for a few seconds. No health points are taken, though.

In Quake II and III not only are enemies just as susceptable to environmental hazards as the player, but they can and will engage in fights to the death amongst themselves. In Quake III that was expected to happen, since there was only a multi-player game and all the NPCs where bots. But in Quake II singleplayer you will sometimes turn a corner to see a Gladiator bullying a puny Light Gaurd to death or Enforcers shooting down pesky Flyers, who evidently annoy everyone including their fellow Strogg. Not to mention the quickly moving Mutant, which attacks everybody without prerogative including other Mutants. Basically if a Strogg meets another Strogg of different class, they might just duke it out, with guns, and explosives.

Strogg of similar types usually get along, but can be provoked against each other. The AI in Quake II wasnt too advanced, but it was advanced enough that if you took a shot at one, hit or no hit, they would probably come by to investigate. If you could enact a scenario in which you took a shot at a Strogg, and he turned or came to investigate and found another Strogg, sometimes they would fight. Sometimes the unit would just look around for a bit then go back to it's patrol route or gaurd position. The Strogg occasionally seem to get frustrated and fight amongst themselves when a group gives chase and you manage to evade.

As a side note: The Strogg units seem to be a bit more tolerant of the Medic unit, for logical reasons. Some of which may be that the Medic has a Gladiator type lower chassis, a Berserker cutting arm, a mounted Tank HyperBlaster, and 300 Health.

By The Way: What is a grown man doing playing "Chip 'n Dale: Rescue Rangers"?

iandunn says the enemies also fought in Doom, another iD game.
Pulsedriven says in Prince of Persia the enemies could be hurt by obstacles
arrogantsob says obstacles also do damage to enemies in Streets of Rage
amib says there was friendly fire in Marathon and that shell projectiles in the Super Mario games where not discerning between Italian Plumbers and Color Coded Koopas, not to mention that in Mario 2 enemies could be picked up and thrown as weapons
amib says Throwing enemies or beating enemies with other enemies is REALLY common. Gunstar Heroes, Double Dragon, Final Fight, almost every side-scrolling brawler...

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