Untrustworthy Behavior

noun a person who engages in deceit or dishonesty, especially for personal gain

• The businessman turned out to be a double dealer, betraying both of his partners.
• I can't trust him, he's a double dealer who talks behind everyone's back.
• Watch out for that guy, he's a total double dealer.
• Her smile was as fake as a double dealer's promises.

adjective showing a lack of trust or loyalty; not faithful

• The faithless politician was quickly ousted from office.
• I can't believe she turned out to be so faithless.
• He's such a faithless jerk, always breaking promises.
• The crumbling old building stood as a monument to the faithless passage of time.