True Happiness

Moshe spoke in a kindly manner to Pharaoh even while delivering a warning of a devastating plaque. G-d warned him to do so as otherwise he might have lived up to the description in Tehillim of the tzaddik who rejoices when he observes G-d taking revenge. Moshe is reminded that what truly makes tzaddikim happy is seeing G-d's attribute of mercy in action.

(Ohr HaChaim)