Let us recount how Xiahou Lie, with the gruesomely unviewable corpse of Nangong Yin in tow, returned to the hostel with a somber expression. The few hundred meters from the hostel's main gate to the chamber where the Eleventh Prince resided took him half an hour to walk, yet he still had not reached his destination.
What a huge blame to bear on his head, ah, the effect on his career!
Regardless, Nangong Yin was of the Royal Family, the Young Prince from the Jing Royal Mansion. Xiahou Lie had been negligent and allowed Nangong Yin to die right before his eyes, so this blame he was destined to shoulder, no matter what.
Moreover, it was the Demon Clan that "accidentally" killed Nangong Yin; one could not simply say that since the Demon Clan appeared within the borders of the Sky White Empire, it was the Sky White Empire's responsibility to take the blame.