To be fair, Mo Han wasn't afraid of Nie Zhenyuan; with his abilities, he could completely make his way to Dragon City without anyone noticing and take care of Nie Zhenyuan and his people.
However, this was a last resort. The Nie Family had stood tall in Dragon City for many years, with numerous relationships and powers backing them.
If he truly attempted to assassinate Nie Zhenyuan, regardless of whether he was successful or not, it would cause an uproar. At that time, as a party who had a conflict with Nie Zhenyuan, he would inevitably be listed as a prime suspect.
It would certainly attract an investigation from many forces, and there might even be involvement from official institutions.
No matter how powerful Mo Han's martial arts were, it was absolutely impossible for him to confront the machinery of Jiuzhou, especially since he had his family, including his wife and children to consider.