In this case, the emperor and Wang Zhenbei conspired to slaughter 380,000 people in Chuzhou, and then lied about the outbreak of the plague as a reason to cover up the crime.
As for why it should be kept secret, it is mainly because the emperor wants to master the military power in this way and enhance his own strength in order to better rule the world.
There are also some political reasons, for example, the emperor wants to get rid of some opponents and make his rule more stable.
In a word, this case involves many aspects such as power struggle, political conspiracy, cruelty of human nature and so on, which can be said to be a very complicated plot in the novel.