How do we think about personal agency?

In describing the valid functions of prisons, Shelby writes that "it is, unfortunately, sometimes necessary to rely on incarceration to incapacitate highly dangerous individuals" (The Idea of Prison Abolition, 52). And while this intuitively makes sense, it makes me wonder what methodologies are valid in determining whether we have the right to judge someone a danger to others. There are the more obvious cases when one expresses intent to harm others, but how do we think about the cases when we incapacitate someone because we think they are lying about their intent? In some way, we must be denying their ability to make rational and moral judgement. Shelby responds to this issue by saying the use "of hard treatment, a form of preventative force, is neither dehumanizing nor unfair, at least not when prisons are humane and courts operate justly. Forewarned by the public legal proscription and equipped with the capacity for rational and free action, offenders had an adequate opportunity to avoid this unwelcome and unpleasant treatment" (61). I think this is difficult because it assumes that everyone was given equal opportunity to rationally think through their actions. If we believe in reform, we must believe that given reflection and education, perpetrators would eventually conclude that crime is irrational. Committing an irrational act could only be explained by the actor either not having the capacity for rationality, or them not having adequate access to information in their decision making. Therefore, if we assume they were forewarned, then we must be accusing perpetrators of lacking the capacity for rational judgement. 

While I think this feels fine when thinking about some comic book supervillain, I think it becomes dangerous when trying to make judgments on actual people. How do you know if someone is genuinely an irrational threat, or if they simply lacked the information to make rational decisions? We often fall back on the idea of "reasonable" judgment, but I'm curious what actually goes into making those decisions. In the case of a repeat offender, it may seem reasonable to say that they lack rational judgment because they clearly had information with the consequences due to their past experience, but how do you know that they won't figure it out next time. We can use statistics and look for behavioral indicators, but I worry that opens up a dangerously dehumanizing can of worms in thinking about society at large. 

There may be a very simple answer here that I'm missing because what we do does seem reasonable, but I'm still curious to hear what that is. 

Have a great day :)

Aidan