Unless I capture a king or a prince, and expect a hefty ransom, I like my generals to be brutal. I usually kill the prisoners more than once a battle, because I like the sound effect it makes.
The ONLY reason I let rebels live (and often the ringleaders) is because it grants the king the merciful list of virtues, and the resulting increase in happiness of the entire kingdom is worth it.
So, let the king be the nice guy, while his generals are out committing attrocities.

Guthwyn