perfidious

tending to betray; especially having a treacherous character as attributed to the Carthaginians by the Romans

Guilty of perfidy; violating good faith or vows; false to trust or confidence reposed; teacherous; faithless; as, a perfidious friend.

Latin perfidus, from per- + fidus, faithful

“Perfidious Snake-plants had strangled what I knew Was a pavilion once: each oak Held on his horns some spoil he broke By surreptitiously beneath Upthrusting: pavements, as with teeth, Griped huge weed widening crack and split In squares and circles stone-work erst.”

— Robert Browning, The Complete Poetic and Dramatic Works of Robert Browning Cambridge Edition

“She is trembling, she is fainting, perfidious souls, deaf to pity!" The soldiers continue to demand Arturo's execution, but the sounds of a herald arriving are heard.”