The prince Lucca was disgusted by humanity's hypocrisy. Not only they easily accepted whatever nonsensical pseudo-proof that non-humans were demons just so they could justify their xenophobia, but when his youngest sister was born as a ryūgen, proving both side of his family had draconic blood. Naturally, his parent tried to have said sister killed, but the prince interposed himself, making it hard to kill the girl. Lucca was so suborn he clearly stated that if anything happened to his new sister, he would kill himself.
Why Lucca dying was so problematic for the royal family? Because the gods weren't happy with their decisions as of late, and cursed them into never having any more son ever. Since only a boy with royal blood could ascend to the throne, it mean that only Lucca could ever hope to become humanity's next king.
Not that it helped the human king, his queen or the human nobility to realize they should stop persecuting non-humans. Instead they decided the best course of action to make the gods lift the curse on the royal family was to exterminates non-humans faster. If you asked Lucca they were all morons, to the point he wished he could change species.
Well at least he could use the commotion caused by the summoned-hero being a ryūgen to escape with his sister. Who know, with luck he might be able to entrust her to a group of non-human willing to take good care of her… He probably won't survive, but death is better than being the only sane human left, he reasoned.