[nkoren]

> Why do criminals need to be treated humanely though?

Because to treat a human being like they are not a human being makes you evil.

> Won't the deterrent effect of prison be reduced if we go too easy on them?

Data says: no. Beyond the simple deterrence of losing one's liberty (which is significant), the harshness of prison has no real deterrent effect -- but does serve to acclimate prisoners to violence, coercion, and anti-social behaviour. What prison does is rehabilitate, or fail to rehabilitate. That is the metric which matters.