Answer:
I will take the side of Zeus. I think he was just in punishing Prometheus for his action.
In the Greek mythology, the Titan Prometheus was a known to be a trickster. He tricked Zeus by presenting him the wrong food to eat. Also, Prometheus stole fire and gave to man.
So, these crimes committed by Prometheus led to the cruel punishments that were meted on him by Zeus. Zeus punished him by tying him up and sending an eagle to eat his liver everyday.
In life, there are punishments for wrong done. It's not injustice if one is punished for the evil he committed. But punishment should be carried out with leniency.
Therefore, I think Zeus was just in punishing Prometheus.