disloyally

/dɪsˈlɔɪəli/
adverb
  1. In a way that shows a lack of loyalty or faithfulness; treacherously.
    • She acted disloyally by revealing her colleague's secret to the boss.
    • The knight was accused of behaving disloyally when he switched sides during the battle.
    • He disloyally shared the team's strategy with their rivals.
Antonyms