From Halopedia, the Halo wiki
Capital punishment, informally known as the death penalty, is a form of legal punishment involving the execution of an individual. The death penalty is used when violating laws of particular importance of the United Nations Space Command, such as the JAG 4465/LHG protocol. Some instances of the death penalty being threatened on UNSC personnel include failure to comply with the Cole Protocol, insubordination and cowardice.
Humanity is not the only species to administer the death penalty. The Covenant punish various infractions of varying degrees with death: captured prisoners are executed once their usefulness has ended; incompetent Unggoy are brutally killed, or even fed to Kig-Yar; leaders responsible for military failures, such as Thel 'Vadamee, are publicly humiliated and executed; and heretics and other dissidents are silently assassinated. The Covenant seem more free in dispensing their form of justice, often without any form of trial other than a public shame inquisition, and use them as opportunities to serve as examples to others who would follow in their footsteps, of the consequences of those who fail or oppose the Hierarchs and their followers. At the end of the Human-Covenant War, the High Prophet of Truth was essentially executed by Thel 'Vadam with an energy sword following the Covenant's defeat at the Battle of Installation 00.
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