Killing a City Lord under normal circumstances is, of course, a significant matter. However, the case of Xi Munan is different.
The source of the plague has not yet been found, but there is no doubt that Xi Munan is the culprit responsible for allowing the plague to spread to this extent.
Whether it be for the sake of his family or his own position of power.
After the full outbreak of the plague, he was despised by everyone in Yang Country, and killing him was what the people desired.
When Jiang Wang drew his sword to kill him, in the eyes of the public, it was an act of justice.
Furthermore, it was through his efforts that the truth about the plague in Jia City was revealed, a contribution that, without need for him to boast, was plain for all to see.