For the good of society at large, it does make sense to isolate some particularly dangerous people from the general population. At the same time, society has an obligation to manage imprisonment in a sensible way, including by avoiding the vindictive temptation to make prisons themselves Hobbesian jungles in which those who are incarcerated have no personal security, and only bad examples to follow. Rather than locking up more and more people in worse and worse conditions, we should lock up fewer and treat them better. The probable result of that is less cost and harm to society, along with a chance at genuine rehabilitation for those who do commit crimes.
Sending non-violent offenders to prison doesn’t really make any sense. This is particularly true when it comes to non-violent drug criminals: a class that includes ordinary users, but also producers and smugglers. Treating drugs as a criminal problem only makes them more problematic for society by making them a lucrative racket for organized crime groups, and by ensuring that those who operate in this business can only settle disputes through violence. As with alcohol and gambling, society should recognize that prohibition causes more harm than good and undertake a transition from a drug policy founded on criminal law to one founded on evidence-based medicine and harm reduction.
Similarly, having prisons in which inmates fear for their personal safety doesn’t make sense. Living with that kind of stress simply has to be harmful to the human mind, and likely to exacerbate whatever issues led to their imprisonment in the first place. When someone is branded with a criminal record and ‘ex-convict’ status, it already becomes hard enough for them to sustain themselves and any dependents financially in the future. Adding traumatic years of fear and violence to that can only worsen things.
Plausibly, reducing the prison population by excluding non-violent offenders could allow for more resources to be devoted to each prisoner who remains. These could allow for greater personal security, through measures like reducing over-crowding, and for genuine rehabilitation programs focused on things like addressing existing addiction problems and developing skills that are in demand in job markets.
The idea that criminals are bad people who deserve to be punished for their wickedness probably belongs in the Middle Ages. As we learn more about human psychology, we learn that people are profoundly influenced by the environments they inhabit and that people respond in predictable ways to circumstances like stress and deprivation. Rather than seeing criminals as wicked individuals who should be expelled from society to the greatest possible degree, I think it makes sense to have a bit more pragmatism and compassion and to establish systems that minimize the harmfulness of crime while giving criminals better options.