From Halopedia, the Halo wiki
Treason is a legal concept referring to the betrayal of a state authority to which one owes allegiance. It especially refers to such betrayal as through making war on the state, giving aid to the enemies of the state, or attacking the head of state. Persons who defect from one state to another may also be considered guilty of treason. Sedition, the inciting of civil unrest or insurrection, is also frequently considered treasonous. Colloquially, treason is often used for any perceived act of disloyalty or betrayal. A person who commits treason is known as a traitor.
Treason is a crime within the Unified Earth Government and United Nations Space Command. Commiting treason during wartime is a capital offence. Major Standish claimed that the penalty is permanent induced coma, with the injection publicly broadcast. According to JAG 845-P and JAG 7556-L, violation of the Cole Protocol is treason and is punishable by life imprisonment or execution. Violation of the confidentiality of certain ONI transmissions is also considered an act of treason and is punishable by death.
The idea of treason is present within Sangheili culture. It is punishable by death. While the High Prophet of Truth expelled the entire Sangheili species during the Great Schism, a few Ossoona had committed actions deemed "traitorous" by other exiled Sangheili and were branded as such by the keep.
List of known traitors