Synonyms: faithless, unfaithful, false, disloyal, traitorous, perfidious
These adjectives mean not true to duty or obligation. Faithless and unfaithful imply failure to adhere to promises, obligations, or allegiances: was faithless to her ideals; an unfaithful servant.
False emphasizes deceitfulness or duplicity: "To thine own self be true, / And it must follow, as the night the day, / Thou canst not then be false to any man" (Shakespeare).
One who is disloyal betrays an allegiance: disloyal staff members who exposed the senator's indiscretions.
Traitorous most commonly refers to disloyalty to a government or nation: a traitorous double agent.
Perfidious often connotes vile or contemptible behavior: "The propagandists also highlighted atrocities committed by enemy soldiers, in order to demonstrate the perfidious qualities of the enemy nation" (David A. Bell).