Might Makes Right

Question: What's the difference in the wrongness between Cronos eating his children and Zeus eating Metis, considering that cannibalism was taboo and Zeus once flooded the earth because people started eating people?

There's only one difference, and it's that Zeus is in charge now instead of Cronus.

I know this may blow some minds out there in the reading audience, but Zeus is not actually a good person, at least not by modern standards. In fact, he's an enormous bastard, prone to serial rape and execution of people he doesn't like. And one of his greatest sins, which not even the ancient Greeks tried to excuse him for most of the time, was the devouring of Metis, the Titan goddess of cunning and quick thinking. After learning that a prophecy claimed that Metis would bear him a son who would become greater than him and take power in the heavens, he swallowed her rather than allow her to carry her pregnancy to term; she gave birth inside his body, and Athena was born from his brow as a result.

And the only reason that shit is okay is because Zeus did it and Zeus is currently the king, so nobody's going to touch that one. His motivation and method are exactly the same as those of Cronus, who ate his children because he feared they would overthrow him, and having narrowly escaped being a victim of that crime, he can hardly claim to not understand how terrible it is. You're right that cannibalism was not at all okay with the ancient Greeks - the myth of Tantalus, who ended up in Tartarus for eternity for trying to feed the gods people, is another example of how it's a big no deal - and that includes cannibalism that is also murder of your own family members, which only compounds the sin.

Zeus knows this. After all, he's the guy who made the law about not eating people in the first place, and who punishes others when they break it. But Zeus wants to hang onto his power and his throne (and, perhaps, his life - this prophecy's not very specific about what exactly is going to happen to him), so, like any other power-hungry king in any other set of stories, he is willing to go to the extreme of murder and even cannibalism to ensure that he keeps what is currently his.

Now, a few caveats. To start with, when Cronus and Zeus swallow people, it's ambigous whether or not they actually kill them or just sort of imprison them in their ironclad god-bellies. Metis was still structurally sound enough to give birth after being eaten (although there are a lot of gross ways that could have happened even if she were dead), and Zeus' siblings were still alive after Cronus vomited them back up. Considering that the ancient Greeks conceived of their gods as thoroughly immortal and unkillable, that may not mean anything; these poor people could have been torn to shreds, chewed up and simmering in digestive juices for centuries and still be alive, and just pulled themselves together after being saved from the stomach. We're not dealing with mortals, which makes cannibalism both less finally dangerous because they'll survive, but also more potentially horrific, also because they'll survive.

What's really going on here is the cycle of kings, a recurring motif in mythology around the Mediterranean (not just in Greece but in the neighboring Mesopotamian, Egyptian and Hittite mythologies as well). The first king, Ouranos, is violently overthrown by his son Cronus; Cronus is in turn violently overthrown by his son Zeus. Zeus is in turn prophesied to be violently overthrown by his son, and, like all his ancestor-kings before him, is desperately trying to avoid that fate, all the while knowing that his father and grandfather couldn't stop the cycle from continuing so his own odds aren't looking great. And if and when that happens, the younger generation of gods will look upon him with the same disgust and judgment that his own does against Cronus, because once he is no longer king, he loses the protection of power and his crimes, no matter what their motivation, are laid bare for all to see. Should this prophesied son of Zeus succeed in overthrowing him and claiming the throne of Olympus, it's almost certain that he would in turn one day have to face his own children rising up against him.

It's an old story: the ones who make the rules are also the ones who can get away with breaking them, so while it is totally not in any way more morally right for Zeus to eat Metis than it was for Cronus to eat his children, no one will ever bring it up if they want to remain unfried. Zeus probably justifies it to himself by saying that his power and sense of justice are needed to rule the universe, fight off the Titans and keep law and order, so Metis' sacrifice was necessary for the greater good.

But, of course, Cronus, who ruled over the golden age of humanity when there was no want, hunger, crime or sadness, probably said the same thing.