I think the duchess is the dragon ... just saying it now
She didn’t ask for any information about the world, the demon race or the reason for the war. She should be suspicious about the situation cause they didn’t talk about trying to end the war peacefully but to just save only “ mankind”. So they might even be the ones who started the war and have prejudice against demons....this is just speculation
Mc acted more like an assassin/ mercenary for hired than a knight to guard and protect people. She should have known the more killings of high profiled people would come with consequences especially targeting the crown prince.