Backstabber

The team lost the game because one of their own players was a backstabber.

We were supposed to work together, but he took all the credit for my idea. What a backstabber.

It's hard to have a close relationship with someone who is a backstabber.

I can't trust him anymore after discovering that he is a backstabber who spreads rumors about others.

The student was bullied by the backstabber who spread rumors about him to the entire school.