"Once suffering changes a person's social status, the resulting vulnerability can expose them to institutions and relationships that interpret their vulnerability as evidence of personal deficiency. That interpretation can justify additional restrictions, surveillance, exclusion, or punishment, which in turn deepen the original vulnerability."

Historically, we can see several versions of this:

Initial vulnerability	Interpretation	Additional harm
Poverty	Idleness	Workhouse / coercion
Debt	Irresponsibility	Imprisonment
Homelessness	Vagrancy	Arrest / forced labour
Unemployment	Laziness	Labour discipline
Colonial marginality	"Backwardness"	Movement controls / forced labour
Illness	Dependency / malingering	Disbelief / exclusion
Disability	Incapacity / burden	Institutional segregation
Social deviance	Moral defect	Criminalization

The common structure is not sadism.

It is moralization of vulnerability.

And once vulnerability is moralized, suffering can cease to be evidence that someone needs protection and become evidence that they deserve discipline.

That is, I think, the strongest historical evidence for the proposition we began with.

An especially interesting next step

There is a still darker version of the question: why do societies sometimes seem to become more punitive precisely when a person's suffering makes them less capable of defending themselves?

That takes us from poor laws and workhouses into Foucault, Nietzsche, René Girard, Hannah Arendt, scapegoating theory, social dominance theory, and the history of "dangerous classes." There is a fascinating convergence there, but also important disagreements about why the phenomenon occurs.