The prince is despicable but I'm not really surprised. If you think about how royals often were in history and mix in political tension, personal ambition, and dubious loyalty of the people nearby, it'd be more odd if he was kind and gentle. Though it's a terrible command, I get the reasoning behind it. He doesn't just want a good a sword, he wants a sword that unquestioningly obeys him. If he can't obey a direct order to strike down whoever the prince sees fit, he won't be of much use in the prince's ruthless quest to kingship which would likely involve a number of bodies and shady dealings.