Here's a question for you: should one always tell the truth under all circumstances? Molière seems to think not. Sure, Tartuffe is the one that does most of the lying, but what are we to make of, say, Elmire? She's willing to lie and cover up the truth – twice – in order to manipulate and expose Tartuffe. It is important to remember that she gets results by lying, while Damis only creates more trouble by telling the truth. This isn't to say that one strategy is better than the other of course. If anything, it speaks to how slippery a thing the truth can become once the lying begins.
Molière makes it clear that deceit, though often harmful, can also be a useful and perfectly ethical solution to a problem.
"[A]ny wrongful act you care to mention/ May be redeemed by purity of intention" (4.5.13). So says Tartuffe when he's trying to seduce Elmire. Strangely enough, Molière doesn't completely disagree with that statement. Sometimes, he suggests, the ends justify the means, and even lies can be used to do good.