For those of you who want to emulate Half-Life, here's how to make monsters mad at one another. You really need to script fights carefully though, you aren't creating alliances between groups of monsters, you're just selectively annoying them as if they shot one another. So after they kill their one target, they'll just stand around unless attacked again, even if others are fighting.

Perhaps a good way to choreograph a bigger brawl would be something like
ogre 1 gets mad at fiend 1
fiend 1 gets mad at ogre 2
ogre 2 gets mad at fiend 2
fiend 2 gets mad at ogre 3
...
fiend n gets mad at ogre 1
that loop should make sure everyone fights.

Anyway, getting ahead of myself, what you do:
Set up your monsters, run the map, type edicts in the console and note down the entity numbers of the monsters you want to fight.

Add a brush over each monster, and convert to info_notnull with the following settings

owner n
use InitTrigger
targetname que1
touch spike_touch

where n is the edict number of the monster you want it to get mad at.

Then two more triggers to set the thing going, a trigger_once with target que1, and an info_notnull with

use teleport_use
targetname que1

In fact, this is such a minor thing, you could even tag it onto the info_player_start you use and save an entity.

Of course, the way it works is that the triggers behave like a nail fired from the monster in the owner field into the target monster, so it gets mad according to the infighting code. You don't have to use trigger brushes, spike_touch and teleport_use if you don't want - voreball and ogre grenade explosions set off from point info_notnulls with owner set have the same effect, if more dramatic in execution. Be warned, this will fail on monsters of the same type - for a minute there I used two knights to test it and thought the hack wasn't gonna work!