What is the ruling on a Muslim cursing another Muslim, and how should one who is subjected to cursing by a Muslim react: should he return the curse in kind, or remain silent and ignore it? And what about being cursed by a non-believer: is it permissible to hit or kill him?

Reviling a Muslim without just cause is forbidden. If one is reviled, the best course of action is to pardon and overlook. If one does not pardon, then there is no harm in retaliating with a similar reviling, provided there is no excess or transgression against an innocent person, and without using forbidden types of reviling such as excommunication (takfir). As for retaliating against the reviling of a dhimmi, a mu'ahid (covenanted non-Muslim), or a musta'man (one granted safe passage) disbeliever, it is permissible to do so in kind without exceeding the bounds.