I always liked this feature, going back to at least the original DOOM, where you could lure enemies into accidentally hitting each other which would cause them to focus on each other. I love it in RPGs, where you could stumble upon guards fighting a dragon in Skyrim, or Might & Magic VII-VIII where monster fights could occur. Or seeing the Covenant fight the Flood in Halo. or STALKER.
Looking for more good examples of games where enemies attack each other, either as a result of you manipulating them to hit each other, or just as part of an ecosystem (prey/predator) or just as a result of animosity dictated by the setting.