Chapter 78, Lay Down Your Arms, Don't Be a Traitor.
Killing Mondok in his current state was as easy for Gu Hang as snuffing out a chick. His spiritual energy could lift Mondok's entire body into the air; the same force could easily break Mondok's neck without any problem.
Gu Hang did not do so; instead, he chose to shoot Mondok, and the reason for an execution by gunfire was the sense of formality.
Gu Hang wanted to use this symbolically significant method of execution by firing squad to tell all the soldiers that their former commander was a criminal, a traitor to the Empire. And they should, at this moment, completely disassociate themselves from the traitor and accept the leadership of the Governor.
He also did not plan to immediately put to use all of these ten thousand soldiers of Revival City. But at the very least, these men definitely could not become an obstacle to his next actions.
If a great purge was inevitable, it would start first within the army.