I'm sorry, but what fatuous statements. If any of us ever go to war with attitude we are not fit to serve. It is not our place to give quarter in the shooting part of any war. It was his job to destroy the ship, without consideration of the enemy, in this case us. It was his duty to employ any means at his disposal to do harm to the enemy. Not to think about trying not to kill the people that were trying to kill him and that he was under orders to kill. His job was to sink the ship and that, potentially, included killing everyone on board.
That's how it is.
All correct but when war finished, he was asked and gives a response, then listen to the response.
He did as he was expected to do for his country, no different from what is expected of anybody in uniform.
Fact is that years later he happy that he hadn't used the cannon to kill lots more is not a sign of regret that he didn't do his job, its a sign that he glad he didn't kill more people.