Because what kind of human can condem a persons cruelty, with cruelty.
In my opinion to want harm to come to another is one of the worst traits a person can have.
We have human rights for a reason and that's because we as a society have realised that cruelty is never justified.
True sometimes you can argue that the ends justify the means, however by making a person suffer there is no possitive outcome.
Prison should be a place of rehabilitation, not petty revenge.