Torturers can choose methods that hurt while leaving no trace, such as covering a beaten body in ice so fresh bruises fade. That turns cruelty into something deniable, and it shows the abuse was deliberate, not a loss of temper.
Guards who carry out abuse often know it is wrong, but go along because refusing would cost them a decent post or a good transfer. Institutions that hand the dirtiest work to their poorest members get compliance cheaply. Yet 'I was ordered to' has never cleared anyone, any more than it did soldiers judged after the Second World War.
Once a suspect has a past link to the wrong group, a web of connections, accusations, and weakly sourced evidence can hold him for years without any charge. The burden flips, so he must keep proving what he is not. Release depends on disproving a story, not on anyone proving a crime.
When many prisoners who never met each other describe the same treatment, their accounts confirm one another. They had no way to coordinate a lie. Matching stories from isolated witnesses are evidence that the system works this way by design, not by accident.
Even under abuse, a prisoner can treat each guard and interrogator as a separate person shaped by fear, confusion, and pressure from their superiors. This refuses to dehumanize the people who are dehumanizing you. It also holds them more responsible, because each one knows kindness from cruelty.