Backstabber noun
Someone who says or does something harmful to you behind your back
There must be a backstabber in our organisation who has revealed our secrets to our enemy.
I would have tolerated her even if she turned into a backstabber.
He played backstabber to me when he talked bad about me behind my back.
People tend to like forbidden or illegal things just because they are forbidden or illegal.
He must have believed that stolen fruit is the sweetest, and he, therefore, cheated on his partner.