Perfidy
Etymology
From Latin perfidia, "faithlessness, treachery," linked to the phrase per fidem decipere, "to deceive through trust": per, "through," and fides, "faith." The construction is pointed — the betrayal works by way of the very trust it violates. Perfidy is not mere dishonesty but the abuse of confidence deliberately given.
noun · /PUR-fih-dee/
Deliberate betrayal of trust — treachery toward someone who had every reason to rely on you.
What it names
The vice of the trusted person who turns traitor, breaking faith precisely where faith was owed.
Example
The general’s perfidy — passing the battle plans to the enemy — cost the lives of the men who had followed him for years.
Related idea
Duplicity is the two-faced deceit that often makes betrayal possible; perfidy is the betrayal itself, the actual breaking of a trust.