I phrased it awkwardly. Simply put, in Ancient Greece, liver was something like heart in modern society. It was the seat of human emotions.

So by destroying Prometheus' liver, Zeus attempted to destroy his human emotions, including his pride and defiance against gods.

It was the believed area of emotion, passion and 'soul'. I think western/modern religion has morphed from the liver to the heart essentially.

There are a bunch of Japanese words (including a several loanwords from Chinese) that use the liver / gallbladder metaphorically for courage. Both derive from associations between the humours and personality / behavior. I have no idea, however, why both Eastern and Western naive psychology associate gall with, well, gall.

Humors perhaps. Not my area, so I do not recall the correspondence ...