disloyal
I would regard it as an act of disloyalty to break a confidentiality agreement.
He showed shocking disloyalty to his old friend.
The coach faced disloyalty from his own players.
He expected unquestioning obedience, and punished disloyalty harshly.

SMART Vocabulary: related words and phrases

Disloyalty, betrayal & treason
adulterous
backstabber
betrayal
betrayer
break ranksidiom
chicane
deserter
desertion
faithlessly
faithlessness
false
fifth column
fifth-columnist
stab someone in the backidiom
tergiversation
throw someone to the lionsidiom
throw someone under the busidiom
traitor
treasonable
unfaithfulness