perfidious

kavala Of, pertaining to, or representing perfidy; disloyal to what should command one's fidelity or allegiance.
1610, s:The Tempest|The Tempest, by w:William Shakespeare|Shakespeare, act 2 scene 2

TRINCULO (speaking about w:Caliban Caliban): By this light, a most perfidious and drunken / monster: when his god's asleep, he'll rob his bottle.

The perfidious Ricimer soon became dissatisfied with Anthe'mius, and raised the standard of revolt.

She knew Huntly for the ambitious traitor he was, a man peculiarly perfidious and self-seeking.

When the Nazis branded Feininger a "degenerate artist" in 1937, he left 54 paintings for safekeeping with a Bauhaus friend named Hermann Klumpp. After the war, and for the rest of Feininger's life, the perfidious Klumpp refused to give them back.