You guys are talking like killing people one-sidedly is fine as long as they "started" it. The power imbalance is severe and obvious. Itami or psycho girl didn't need to kill almost everyone there or torture (because that's what that was, and I'm pretty sure this is not allowed even in RL, openly at least) the prince to make that point. I'm not saying it's completely ridiculous because it does leave a lasting impression. But it is definitely not the humane thing to do.

Japan has been doing this all along, but it was kinda disappointing to see Itami being no different.