Zhou Tian was able to smoothly expand his territory before mostly because his strength was extremely formidable, so nobody could stop the expansion of his strides at that moment. But now things were different; the emergence of another Human Emperor Monster signified that Zhou Tian had met his match.
Although Zhou Tian was undoubtedly the strongest monster in this world before, the appearance of another Human Emperor Monster meant that even if the opponent couldn't defeat Zhou Tian, he still possessed the ability to duel with him. Given the 'Immortal Body' ability of the Human Emperor, once someone claimed that title, it was destined that it would be very difficult to determine a winner among equals.
Therefore, even though Zhou Tian had been psychologically prepared beforehand, when he suddenly found that the excellent situation of unifying the world was threatened by another Human Emperor force, his mood could not possibly have been good.