Disabling the monsters from killing each other

Is there any way that I can disable the monsters from killing each other when they get hit by another monster?

Zdoom has the dehacked option of

Monsters Ignore Each Other = 1

If you put that in the misc section of your dehacked patch, the monsters will not infight. Projectiles from monsters will not hurt other monsters. Hitscan attacks from monsters will still damage any monster that gets in the way, but the damaged monster will not retaliate.

Do you sympathize with the monsters...? Terrorist!

MBF, and thus also PrBoom and Eternity, have a menu option that allows what ZDoom does through DeHackEd (or ZDeHackEd, really), in the Enemies section. This can be applied to a wad automatically by enabling the option in an OPTIONS lump within the PWAD.