Zhou Tian was well aware that, with his abilities, he had no way of preventing that Human Emperor from recovering from his injuries, which is why, from the very beginning, he had not entertained the same aggressive approach as his adversaries, aiming to forcibly Kill the enemy. Given the Human Emperor's abilities, to Kill them in the Main Plane would hardly be easier than destroying this world.
Therefore, if one truly wished to Kill the Human Emperor, it would still be necessary to approach from an alternative method.
For instance, the simplest method would be to cause the Human Emperor to fall from his throne. Once he was no longer the Human Emperor, the effects of his Immortal body would naturally disappear. Based on his own strength, with the Human Emperor's abilities, competing against Zhou Tian for victory was simply an impossibility.