Damis jumps out of the closet – remember, he's been hiding in there the whole time – and contradicts Elmire's promise.
He wants to tell everyone about what a liar and a sleaze Tartuffe is; it's the second best thing to punching him in the face.
Elmire tries to do some damage control and convince Damis to keep his mouth shut; she wants to honor her promise and avoid upsetting Orgon.
Damis can't imagine letting Tartuffe get off so easily. He, who has seen his marriage prospects nearly disappear, craves justice, and he's just had the perfect opportunity served up on a silver platter.
Elmire's attempts to calm him down fail; he wants only revenge, and he intends to get it.