Zeus was an idiot
He lets Thetis have revenge on the people of that town just because Perseus wounded Calibos in self-defense, and because she felt slighted that some mortal thought Andromeda was more beautiful?
shareQuite the contrary, throughout the film Zeus' actions are in accordance with a harsh sense of justice. Thetis makes it clear she can do nothing to avenge Calibos because Calibos is hated by Zeus while and Perseus is his favorite. Zeus would not tolerate private, petty vengeance against his son. Then Cassiopeia gave Thetis a door through justice.
Impiety (disrespect for the gods) and hubris (excessive pride in the form of comparing yourself to the gods) were major crimes against the Olympians. Worse, Cassiopeia made the insult in Thetis' own temple. Thetis publicly addressing the insult was the final nail in the coffin.
If Zeus did nothing and let the insult pass it would have made a liar out of Thetis, cause a loss of respect for the gods in general implying you can insult them and get away with it, and undermined Zeus' rule by making him lose face in front of the other gods. As a son of Zeus, Perseus could get away with defeating the Kraken because in its own way it brings glory to Zeus. Insult the gods and you pay the price, but Zeus in his mercy gives you a chance to be spared. Ultimately, life and death are shown to be controlled by Zeus.