This magnanimity is also something prized in Earth-medieval kingship, too. Many a revolt against a king ended with the king pardoning the vassals that rose up -- only a king very secure in his throne could depose or even execute rebellious vassals. Of course, it's important that it be a common member of the nobility for this to work consistently. Against commoners, brutality was far less politically costly. And heretics (read: Kathars), who risk the Wrath of God coming over the whole community, are even less worthy of protection.