Despite all evidence to the contrary, Prometheus Bound seems pretty convinced that Zeus is the guardian of justice. Even Io, who seems to have been treated more unfairly by the gods than anyone else, still sometimes clings to the idea that Zeus enforces justice—and that she therefore must have done something wrong to earn the suffering that is now her lot. But everyone also seems to agree that Zeus's judgments are not motivated by any higher principle than his own whims and desires. And that sounds basically contrary to justice. If this is true, and Zeus is simply a tyrant, does this make the one opposed to Zeus—Prometheus—the true guardian of justice? And what does justice even mean when there's just one almost-all-powerful being handing out lightning bolts and punishments?
Io suffers the most unjust punishment in this play, because it's clear that she didn't do anything to harm anyone.
If Zeus is just, then justice seems to involve punishing anyone who goes against the will of the gods, or shares the gods' authority with humans.