Powerlessness

The "retribution" rationale for punishment is often conceived of as an issue of reciprocity of harm -- and eye for an eye, you should feel as much pain as you caused me. But I think on the level of motivation, there's more to it than this sort of hedonic calculus. The basic impulse seems to come from the question of power.

One of the strongest moral desires people have is for agency. We want to shape our own sense of self and life-project (even if we do so by giving ourselves over to God or some group). Most crimes or injustices are premised on a loss of agency, a loss of power over who the victim is and what they can do. The frightening part of being mugged is as much the realization that you're not in control of the situation and that someone else is dictating to you as it is about having less money at the end.

Retribution, then, is an attempt to reestablish the victim's power. It's a way of proving you haven't lost your capability for agency while making your erstwhile dominator into a non-agent, rendered powerless over his own life and liberty.

This may explain some of the appeal of prison rape as an ultimate punishment. Regular prisoners retain some amount of dignity and agency, despite having their life so circumscribed by prison routine. But being raped invades one of the most private elements of the self, stripping the victim of control over something that should be a fundamental act of self-actualization. If "Bubba" can rape you, he's in total control. What's more, it's arbitrary power, regulated by Bubba's whims rather than the (paradoxically humanizing) regularity and predictability of a set of official prison rules. It's this total disempowerment that seems like the only fitting way to reestablish justice after someone has committed an egregious abuse of power.