I literally searched for kotowaza about betrayal

獅子身中の虫
(Shishi shinchuu no mushi; “A bug in a lion”)

Definition:

Someone ostensibly a member of an organization who nonetheless causes it harm. Someone who rewards kindness with enmity. Like a parasite living within a lion and protected by the lion’s strength, yet eating out the lion from within until it dies. Or like a certain political party within a nation, protected by that nation’s strength, yet gutting its government and taking life-saving services away from its people in order to make a handful of plutocrats richer. Treachery from someone who was supposed to be on your side. Ugly, hypocritical betrayal.

Breakdown:

We begin with the noun 獅子 (shishi), “lion,” followed by the noun 身中 (shinchuu), “within the body.” This is connected by the associative particle の (no) to the noun 虫 (mushi), “bug,” in this case referring to parasitic vermin.

Notes:

It’s possible to use the first four characters of this phrase as a stand-alone yojijukugo. A longer version of the saying gives the mushi an action, 獅子を食らう (shishi wo kurau), “eats the lion.”

Pronouncing 身中 as shinjuu or rendering it as 心中 (“inner thoughts” or “double suicide”) would be considered an error.

Today’s kotowaza comes to us from the Brahmajala Sutra (梵網経, Bonmoukyou) of Mahayana Buddhism.

Example sentence:

「この獅子身中の虫等を明日の朝すぐ払い除けても、もう遅い、遅すぎる。来年が待ち遠しい！」

(“Kono shishi shinchuu no mushira wo ashita no asa sugu harai nozokete mo, mou osoi, ososugiru. Rainen ga machidooshii!”)

[“Even if we drive out these traitorous parasites first thing in the morning it will be late, way too late. I can’t wait for next year!”]