HARM THY BROTHER
Before Doom's source code came out there was DeHackEd, which allowed a number of changes to be made to the game behaviour. One of the more interesting ones was called "Monsters Infight".
When the source was released obviously DeHackEd no longer worked, as it was tied to specific binaries of vanilla Doom. The solution was that the ports had to load DeHackEd patches themselves and modify their own internal tables. Boom did this and also extended the format a little, making the patches easier to write.
However, for some reason, the Boom developers glossed over the Monsters Infight option. Some sort of a misunderstanding occurred. Boom - or possibly MBF, I'm not sure exactly when this was implemented - does have an option in its menus called "Monster infighting", but it is not the same as DeHackEd's, not at all.
Boom's monster infighting is the literal interpretation of the term. It asks
If a monster takes damage from another, will it consider stopping its chase after the player, and instead chase after its attacker?
The answer to this question is overwhelmingly "yes" - inducing monster infighting is a standard tactic when the player is low on ammunition, and the game can become significantly more difficult if you turn it off.
DeHackEd's monsters infight option has a subtlely different effect. Namely
If a monster is hit by a projectile launched from another monster of the same type (barons and hellknights count as the same type), will it take damage?
The answer in this case is certainly "no" - if you get a big crowd of imps or whatever all shooting at you they don't take damage from each other. Turning this option on in vanilla Doom or a port that supports it makes gameplay a lot easier.
Note the subtlety in the difference - Boom infighting is just that, whether or not a monsters will go after an attacker, if damaged. The monsters can always damage each other, except for in the most common case of two monsters of the same species accidentally shooting each other - and in that case, DeHackEd allowed you to choose whether or not these attacks were felt. Nothing said about whether or not they were acted upon. Also note that it only applies to projectiles - all the other ways monsters have to attack each other, bullet weapons, etc. - will happily cause damage anyway.
Hence, when I put this into rboom yesterday, I called it "monsters_harmbrother". I tried to come up with something with some fancy latin or greek prefix that meant "self" or "same" but no dice, sadly...