Perfidious

The most perfidious way of harming a cause consists of defending it deliberately with faulty arguments.

By agreeing to integrate the "Tamazight language" into its educational system, the perfidious and calculating Algerian state wants to annihilate the Kabyle language by creating a diglossia which will in turn create serious learning problems and cross-comprehension issues. Because this "Tamazight language" is full of neologisms, in addition to being an absurd mixture of several Berber languages, that often have no meaning and has no connection with the sociolinguistic reality for all Berbers. In the end, they all lose out.

Perfidious Albion!

I discovered Troy's perfidious plans.

TRINCULO (speaking about 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.

[S]he 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.

Enraged, the 2010 Nobel literature laureate thundered that he was “flabbergasted to learn that this kind of gossip can find its way into a respectable publication such as the Book Review” - a “slanderous and perfidious” instance of the convergence of posh and pop that his book inveighs against.