Legitimate defense, he was threatening to (and had the means to) kill aound twenty people while in the middle of assaulting a Defenseless Old LadyTM, not to mention the hostile action taken against a body of dwarfish government. And they had no mean to safely restrain him. That's an open-and-shut* case.
*That means easily winnable, right? Like you open the case and immediately shut it because one perty has clearly no leg to stand on?