The thing is, from how this kinds of stories go the "Hero" probably has a very monochromatic "with me or against me" view of the world. So if he's not gonna suck up to him, MC has to be prepared to be his enemy. And don't forget its explained to us that the magical ability of MC's family allows the heir to become more powerful the more he's feared and hated + his family already has a terrible reputation, so why not take advantage of that and kill two birds with one stone?