In Hesiod's Theogony, Prometheus is basically a petty trickster who plays a pointless trick against Zeus, the most powerful god, merely to win humans a better share of food (T&D pp. 397). However, in Aeschylus' Prometheus Bound, Prometheus is a noble character who, through his actions, greatly improves the lives of the humans while suffering for these actions. This is a very big paradigmatic shift and the time difference between when these two works were written (Theogony in about 700 B.C.E. and Prometheus Bound in about 456 B.C.E.) is most likely responsible for this. .
We also see some interesting examples of symbolism in the stories. In both versions of the story, Prometheus is threatened by Hermes to reveal the information he claims to know about the mother of Zeus' son who will depose his father. Hermes says that if Prometheus does not cooperate an eagle will eat his liver every day, and that this will endure until a god agrees to die for him. The eagle traditionally symbolizes courage, and it it ironic that Prometheus' courage to defy Zeus would earn him eternal punishment as the eagle eats out his liver day after day. The eagle is also a symbol of Zeus, and hence Zeus might be suggesting that he himself is taking revenge against Prometheus for his personal against him. In addition, the Greeks believed that the liver housed emotions, and by saying that the eagle would feast on Prometheus' liver, the story suggests that Zeus is directly attacking Prometheus' source of emotion that led him to trick Zeus and give the humans fire in the first place. Finally, it's ironic that Prometheus, whose name means 'forethought', was unable to foresee his fate of being chained to a rock for eternity before he helped the humans. He says that he must live with his suffering because no one can fight fate.
Prometheus suffers this fate because of his choice to help humanity and his defiance of Zeus and summarizes everything he has done for humanity as justification for his actions.