Criminal prosecutions of MBS and other Saudi officials could be brought under the U.N. Convention against Torture, to which Saudi Arabia is a signatory. The convention prohibits acts that inflict “severe pain or suffering … inflicted by or at the instigation of or with the consent or acquiescence of a public official or other person acting in an official capacity.”
Through the principle of universal jurisdiction, any country that is a party to the convention, including the United States, could refer a case to the International Court of Justice and seek an order for Saudi Arabia to prosecute or extradite MBS and the other suspects. A recent example is when Belgium brought a case against Senegal seeking the extradition and prosecution of Hissene Habre, the former president of Chad, for crimes against humanity.
The United States and the Trump administration have resisted the principle of universal jurisdiction, for fear American officials could face charges. But other countries, such as Germany, have been more aggressive in prosecuting crimes such as torture and forced disappearance no matter where they occurred. In June, Germany’s chief prosecutor issued an international arrest warrant for the head of the Syrian Air Force’s Intelligence Directorate on the charge of war crimes.