Why is cannibalism not an evolutionary stable situation?
Cannibalism is incredibly common in nature. There are more species that eat their own kind than do not. Chimps eat other chimps. Lions eat other lions. Wolves eat other wolves. Prairie dogs go into the burrows of other prairie dogs and eat their babies to get the protein-rich meals needed to produce milk for their own litter. The only animals that don't regularly engage in cannibalism are some birds, and that is more because they physically cannot fit other members of their species down their throat than any moral qualms.
However, cannibalism is typically a bad evolutionary strategy, long term. For several reasons...

in a dioecious species (i.e., one with two sexes), cannibalism means a 50% chance of eating a potential mate and losing the opportunity to pass on your genes
diseases are more likely to transfer between species the more closely related they are. If you are a cannibal you are eating a species with a 99% similarity to you and are more likely to get diseases or parasites. Like kuru/mad cow/chronic wasting disease.
it precludes social behavior and all its benefits. Why engage in mutually profitable behavior when you can just eat those around you? This is thought to be one reason why pack-hunting is rare in some groups of vertebrates like monitor lizards.
on a broad-scale evolutionary perspective, cannibalism means removing members of your own population from the gene pool. It may be good for the individual but it's bad for the species as whole
cannibalism means you are feeding on an animal that is potentially your own size. That's a bad strategy in general. Most cannibalism in nature tends to be adults eating infants or adults scavenging carrion of their own species. By contrast predators are normally larger than their prey, with a few exceptions like pack-hunting wolves and lions.