That was another fragment of the demon king, his blood. Van simply integrated it without any issue, so it was glossed over. It would have been impossible for anyone else. Why is the real question that OP was asking.
It's been so long I barely remember, but it's mostly that the demon king was , so being one lets him fool the fragments, combined with his insanity and which lets him resist all mental corruption.
Yuuji's cluelessness over the good result he produced was infuriating. "It's not exactly what I was looking for. It's something a million times better. I guess I failed..."
At least, he had better instincts for that land grab scheme.
He almost got his gf killed by being reckless, so what does he do as soon as he can? Be even more reckless, while being unable to use magic. At this point, he deserves to die.