What is the ruling on a Muslim who killed a disbeliever and mutilated him for refusing to give him money, and what are the consequences if the money was rightfully his or not rightfully his?

What the young man did, killing, is a grave sin and a tremendous crime. Killing a soul unjustly is forbidden. The slain disbeliever who is not at war (the one with whom a covenant has been made, the dhimmi, and the peaceful one) has his blood, property, and honor protected. If the property belonged to the slain, then the killer's sin is even greater. Scholars have differed on the retribution (qisas) for a Muslim killing a disbeliever. However, the Malikis and those who agree with them believe that whoever kills by treachery (ghilah), for money, is to be executed as a prescribed punishment (hadd) and qisas, even if the slain is a disbeliever and the killer is a Muslim. There is no pardon for this type of killing. The young man must offer sincere repentance and pay the blood money for the slain. If the hidden money belonged to the slain, then his heirs have the right to claim it.