A Shady Person
A shady person is someone who laughs in your face while they stab you in the back with a butter knife.
My coworker is a shady person. He said he'd help me with my project, then stole my ideas and got promoted.
That shady person at the party said he'd bring pizza, then showed up with a bag of chips and a lie.
He smiled at me, then backstabbed me with a fork. Classic shady person.