stab someone in the back idiom
在（某人）背后捅刀子（指伤害信任自己的人）
to do something harmful to someone who trusted you
He had been lied to and stabbed in the back by people that he thought were his friends. 他曾经被一些他视为朋友的人欺骗过，在背后被捅过刀子。
See also: backstabber