Albert Barnes on Matthew 18:34

Delivered him to the tormentors. The word tormentors, here, probably means keepers of the prison. Torments were inflicted on criminals, not on debtors. They were inflicted by stretching the limbs, or pinching the flesh, or taking out the eyes, or taking off the skin while alive, etc. It is not probable that anything of this kind is intended, but only that the servant was punished by imprisonment till the debt should be paid.