disloyal
adj.behaving in a way that shows you do not support or care for someone or something you are supposed to. You use this when someone breaks a promise or betrays a friend.
adj.behaving in a manner that shows a lack of support or fidelity to a person, group, or cause. Often used to describe actions that violate a sense of duty or trust.
“He felt disloyal when he told his friend's secret.”
“The manager was fired for being disloyal to the company by sharing confidential plans with a competitor.”
“The politician faced intense criticism for his disloyal behavior, which many viewed as a betrayal of the very principles he had once championed.”
From Middle English, from Anglo-Norman desleal, desloial, equivalent to dis- + loyal.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.