People remember directives better if you give them concrete examples. Don't be afraid to make memorable examples of disloyal soldiers, disobedient prisoners, or recalcitrant conquered civilians. Think visually!
Clear communication is the key to effective delivery of your point of view. A demand for surrender will be more effective if carried to the enemy by a headless corpse of one of their own tied to a horse.
Remember that every decision you make contributes to building your brand. You'll never be known as "the Butcher" unless you make a conscious choice to chop a lot of meat.
Listen first before you act. Your subordinates have key information to share with you, so it's important that you give them a chance to speak for themselves. That's the only way you'll know which ones are insubordinate and in need of execution.
Set clear objectives, hold people accountable, then get out of the way. Delegate appropriately: there's no point in getting yourself killed—that's what subordinates are for. Remember, it's their job to take that fortified bastion, not yours.