The antonyms for the phrase 'were loyal to' are varied and include: unfaithful, disloyal, treacherous, false, untrue, faithless, backstabbing and traitorous. Each of these words are used to indicate a lack of loyalty, commitment or support. For example, someone who is unfaithful is not loyal to a specific person or group, while someone who is backstabbing might pretend to be loyal but ultimately betray their trust. On the other hand, being loyal means showing steadfastness, devotion and support to a person or cause. In general, loyalty is considered an important trait, and those who possess it are often admired and respected.