Stab in the back

To cause harm knowingly & intentionally to someone like a friend: Usually carried out via - Betrayals of Trust that was given on previous agreements, - Deceitful actions or other forms of lies, - (and) secretly acting in ways that are contradictory to whatever impression you intended to project for friends beforehand. An example would be something like hypocrisy with friends: Pretending to see gossip as bad, only to then have the intention of using something against a friend at a later moment.