Hugo is describing a kind of moral decay that feels like a choice repeated until it becomes a personality. The “crab-like” metaphor works because it suggests sideways motion,movement without growth, progress avoided on purpose. In the novel, some people don’t learn from suffering; they learn how to exploit, how to sneer, how to shrink their empathy. “Employing experience to diminish themselves” is the chilling center: every lesson becomes an excuse to become smaller and meaner. The emotion is Resentful because the voice is angry at cowardice that hides inside bitterness. It also warns that pain doesn’t automatically make someone good; it can also make someone predatory. If you’ve watched a person become crueler over time, this quote gives language to that slow, ugly slide. It’s a dark mirror held up to the idea of “hardening” as strength.