Criminals feel threatened and intimidated by staff in a prison.Is this not a good thing?

Prisons are correctional centres and to be sentenced to spend time in them is the punishment recommended by law. They are not meant as places of suffering, the idea isn't revenge. Even long-term convicts should be treated humanely, not "victimised, threatened and intimidated" by staff, that makes the society that is punishing them just as bad as the criminals.