The punishments of Hell are just and merciful. No fallen angel or reprobate soul can act with justice and mercy, so the punishments are meted out by God. First punishment: deprivation of all the joys of Heaven. God does this by the very act of sending the soul to Hell. Second punishment: the worm of conscience. God does this by giving the soul infused knowledge of their life: good and bad acts are understood by the person with correct moral judgment. Then, in Hell, the person is tormented by the certain knowledge that they deserve Hell because of their own free choices, despite all the opportunities to repent and do good. Third punishment: active sufferings for some of the damned, of a type and to a degree fitting to their particular mortal sins.
The souls in Hell also suffer by having the ugly companionship of other condemned souls and fallen angels. The devils do not torment the souls in Hell actively, as if they were assigned by God to punish. But the souls suffer from the presence of devils as fellow prisoners in the prison that is Hell.
One of the mercies of Hell, which might also be considered a type of punishment, is that the prevenient grace of God prevents all the devils and souls in Hell from ever sinning again. It would be unjust to allow the devils and souls there, billions of them, to continue sinning gravely forever. First, it is contrary to the justice of God to tolerate grave sins unceasingly. Second, it would be unjust not to punish the devils and souls for this new sins (if permitted), and then the punishments of Hell would increase unceasingly, becoming so extreme as to be unjust. Therefore, the only just option is to prevent the souls and devils in Hell from sinning any more. This is a mercy, as it keeps them from deserving more punishments, but it is also in itself a punishment, since their free will is restricted from doing things (sin) that they would prefer to do.
Ronald L. Conte Jr.