In calling him a bully and a coward, I addressed Castro’s moral character, and expressed a character judgment. That has bugger all to do with retributive justice.
In fact, one of the aims of a legitimate, socially constructive penal system (which we do not have in this country) is that it offers those convicted of crimes the opportunity and resources to reform their character, to become better human beings. As you said, rehabilitation should be our aim, not retribution — and what is rehabilitation but character reform? But even in American prisons, for all their flaws, some people have used their time in prison to face their own past — not just their criminal actions, but the life history that led up to their criminal actions — and sought to overcome their problems and confront their guilt. In short, they’ve tried to become better human beings, tried to rehabilitate themselves. But becoming a better human being is difficult, and Castro appears to have been too much the coward to even contemplate such a struggle. Instead, he denied responsibility for his actions repeatedly, then escaped the consequences of those actions at the earliest opportunity. Thus, even the last action of his existence was morally blameworthy.
As Hume pointed out a few centuries back (in Enquiry, Section VIII, Part II), the whole business of making moral judgments absolutely requires that people’s behavior is caused by their character. When we judge that an action was not produced by a person’s actual intentions and predispositions — for example, in a genuine accident — we don’t assign moral blame. The fact that character is itself the result of a causal chain in no way negates the possibility of making moral judgments, or sensibly using concepts such as responsibility and blame. Were there reasons why Ariel Castro was a loathsome human being? Logically and psychologically, there certainly must have been: People develop, they don’t just spring into existence wholly formed. Do the causal forces that shaped Ariel Castro into a bully, a coward, and overall vicious person (in the classical sense of vicious, as opposed to virtuous) somehow expiate his responsibility for his actions? Not in the slightest.
Holding people responsible for their actions is not the same as retribution, and some of your arguments here seem to confuse this vital distinction.