Chazal condemn Esav by making him ask misleading pious questions, such as how to take maasar on straw and salt. They make him into a villain thereby, but that wasn't the point, to make Esav and his descendants (Edom) bad.
The point was to take the tzayid befiv ("he hunted with his mouth") and direct it inward. The homiletic point is that we, Israelites, should take care not to project a false piety in our words and actions. If we think something is not halachically required or halachically efficacious, and doesn't add to our spirituality, we shouldn't do that thing in order to appear "frum". Our religious words and actions should be genuine, because they accomplish something spiritual, or moral in the world or in ourselves. Not to impress others.
(parked from a Facebook post.)