I agree with KrayZ. Naofumi already knows the government hates his guts for reasons he doesn't understand, so there's nothing there he would need to learn from those goons. When soldiers try to kill a princess (or a prince), it's obvious it's for political reasons, nothing there to rack your brains over. Making it happen in front of Naofumi, the hated shield hero, would be self-explanatory. At the end of the day, Naofumi is always waiting for the next ill deed against him from the king/government and he has long since given up trying to ask why. His only concern is how to deal with it. However, to maintain his sanity, sense of personal justice, and pride, he doesn't want to make himself the villain the government calls him. That's why he doesn't attack random soldiers and guards, apart from self-defense. It wouldn't really help his case. Attacking them would only help his enemies. He would rather have absolutely nothing to do with them, which is why he treated Melty like he did.