the.com/backstabber

means Someone who betrays a person who trusted them, especially by attacking their reputation or interests secretly while pretending to be a friend.

from A grim, literal image turned figurative: stabbing someone in the back, rather than facing them, is the coward's and traitor's blow — you can't see it coming, and the attacker never had to look you in the eye. The literal act is ancient, but the figurative use of 'stab in the back' for betrayal (and 'backstabber' for the betrayer) is largely a 19th–20th century English development, gaining extra political weight from the German 'Dolchstoß' ('stab-in-the-back') myth used to explain away a military defeat. From there it settled into everyday use for the office schemer or false friend — no dagger required.