I am honestly puzzled as to why the infighting feature was added at all. It seems that it makes certain monster types intrinsically hostile to other monster types, forcing them to attack one another whenever they happen to be nearby. This is completely different from monster infighting in games like
Doom, where certain types of monsters will retaliate if and only if another monster accidentally hits them (while targeting the player).
I can only imagine that since monster AI in DFU is more efficient compared to vanilla, this somehow mitigates the effect by thinning out the monster population in a dungeon without the player's interference -- but it's still kind of a very odd approach, even if that assumption is true. There is very little sense in removing a part of a dungeon's population without the player even doing anything -- especially since, IIRC, you get a message about every monster that dies regardless of how close you are to that monster (presumably this was done originally to inform the player of any monster that dies off-screen from falling damage or continuous damage spell effects).
It seems that some players do like the infighting mechanic, but personally I feel it adds nothing to the game and does not improve the playing experience in any significant way. It could probably work as a part of a very substantial overhaul of how combat generally works in the game and how dungeons are populated with enemies. But I'm not sure that even the
Doom kind of infighting would make much sense with certain enemy types -- after all, in
Doom there are zombies and demons, while I doubt that human NPCs in DF would be that prone to attacking one another if they're supposed to be on the same side (e.g. guards in a castle or bandits in a hideout), even one accidentally hits another while trying to get at the player.
I can imagine scenarios where you would run into two or more clashing factions in a dungeon -- but this probably requires a different kind of dungeon layout, and much more advanced NPCs/enemies. Theoretically, there could be rival adventurers exploring a dungeon and fighting whatever they come across just like you; or a knightly order being besieged in their castle by orcs or necromancers or whatever; or a jail where inmates break out and attack the guards. But for this to work well, you'd need to rework the dungeons to actually look like such places where the action of this kind would make sense, and also rework the enemy AI so that they would be able to handle this one their own, or support advanced scripts where a castle siege or a jailbreak would actually look like what it is supposed to be.
The current way of doing infighting feels very superficial. Sure, there's some general logic behind "animals always attack the undead because the undead are unnatural", but let's say you have a necromancer's stronghold with bats and skeletons? Or a Vampire Ancient that commands both bats and the undead? Or perhaps you have a coven of druids that have bears and spriggan allies? You can look up the
dungeon types in vanilla DF, and very obviously the designers mixed different monster types to create different themes, and it was clearly intended that all monsters in a dungeon are on the same side
Especially the infighting part, someone on Reddit told me the same. Didn't actually know that, last time I played the OG DOS version was yeaaaars ago.