What Prior feels are feelings of guilt, and not simply regret that things didn't work out differently. He feels the awful weight of self-indictment, the empathy with the victim and survivors, and the need to make moral repair. If he didn't feel that, we would probably think less of him as a commander.
It is more reasonable to feel regret, rather than guilt, about events one does not control.
Guilt demonstrates the eithical quality of leadership in a way that regret cannot.
Feelings of guilt can be awful and make being a commander difficult.
The need to make moral repair is the same as a feeling of regret.