This character commits an act of betrayal.
Betrayal (or backstabbing) is the breaking or violation of a presumptive contract, trust, or confidence that produces moral and psychological conflict within a relationship amongst individuals, between organizations or between individuals and organizations. Often betrayal is the act of supporting a rival group, or it is a complete break from previously decided upon or presumed norms by one party from the others. Betrayal is also a commonly used literary element and is often associated with or used as a plot twist.
When a person betrays their country use the treason trait.