double-cross

我以为他是朋友，但他却背叛了我。

I thought he was a friend, but he ___double-crossed me.

他假装是朋友，实际上却在双面背叛我们。

He pretended to be a friend while secretly ___double-crossing us.

她假装是我的朋友，却在背后捅我刀子。

She pretended to be my friend but ___double-crossed me behind my back.