To display how vicious, ravenous, or just plain evil a predatory species is, writers often have the strong among them literally prey upon the weak. While cannibalizing outsiders or eating sapient creatures is nasty enough, doing so to members of one's own social group is indicative of betrayal as well as cruelty, suggesting an evil or Horror Hunger so indiscriminate that no one is exempt.

With non-sapient creatures, Monstrous Cannibalism conveys how voraciously hungry the predators are and may indicate that their species as a whole is starving and desperate. For sapients, it demonstrates a ruthlessness that bodes ill for any other species they interact with: if their instincts or culture make eating their weaker fellows a standard practice, what hope might a stranger have of being spared? In either case, seeing them turn upon their own for food is often what pushes a species over the Moral Event Horizon into being monsters in the eyes of the audience, not just beasts or antagonists. On the other hand, insofar as an animal could be perceived as monstrous, cannibalism is very much widespread in the animal kingdom.

In a way, this is a parallel trope to I'm a Humanitarian and to Cannibal Tribe for monsters instead of humans, if the tribe eats its own as well as enemies/strangers. Compare To Serve Man, in which victims from outside one's species are preyed upon, and No Party Like a Donner Party, in which cannibalism is an abhorrent last resort. Contrast No Zombie Cannibals and Ape Shall Never Kill Ape.

As a general rule, different types of monsters are more or less likely to be depicted as having these eating habits. Big, bestial creatures, such as dinosaurs, other Prehistoric Monsters, and dragons, are particularly likely to be depicted as devouring their own kind, particularly since even sapient ones are likely to be depicted more as monsters than can talk than as "real" civilized sapient beings. Insofar as prehistoric life is concerned, this is also used to emphasize the idea of a prehistoric world that was more primal and savage than the natural world of today — thus, while dinosaurs and older creatures are often depicted as being cannibals, Cenozoic mammals and birds rarely are. Ogres and trolls are also likely to be depicted in this manner, although due to their humanoid natures the line between "regular" and "monstrous" cannibalism can become a little fuzzy. Vampires are an unusual case in that they're usually not depicted as cannibalistic by default, but vampires who do take to drinking the blood of their unliving kind often become terribly powerful monsters that even other vampires dread.

May result in a Food Chain of Evil within the group. Overlaps with Evolutionary Pressure Cooker if the monsters are deliberately trapped together in order to induce this and strengthen the winners. A common way for the Monster Lord to establish its dominance over its underlings. Sometimes overlapped with Always a Bigger Fish. Done pragmatically (particularly to an individual one deems to be a rival or traitor) it can also overlap with Eating the Enemy. See also Monsters as Cuisine, for when non-monsters decide to chow down on monsters.

As this trope occurs near-universally in real-world carnivorous and omnivorous animals (including desperate humans), No Real Life Examples, Please! There are far too many examples to bother listing.

Examples: