Just as a quick caveat, cannibalism is a pretty fundamental human taboo, so I doubt this is truly the oldest example, but Greek is the oldest my expertise goes.

Hesiod's Astronomia which dates back to the 8/7th cent BCE tells the story of Callisto. She and Zeus have a child together, which her father then feeds to Zeus. The text makes it pretty clear that this is a spiteful act on the part of Lycaon and is not going to end well. Unfortunately, we don't actually have the full text, but later tradition says that he is punished by transformation into a wolf, animals that had a particularly negative reputation in a shepherding-based society. There's good reason to think that this punishment goes all the way back to the earliest stories, since Lycaon is pretty much literally "wolf guy" in Greek.