stab sb in the back

stab sb in the back (to betray somebody; to do or say something that harms somebody who trusts you)
— букв. всадить нож в спину; поступить вероломно; нанести предательский удар; | злословить за спиной у кого-л.

Example: I thought we were friends! I never expected you to stab me in the back.

Example 2: You're firing me after all I've done for the company? You're really stabbing me in the back!

also
[hit below the belt]
[foul play]