Rule number twenty-one of Buyi Town: [One need not adhere to the rules of Buyi Town if they leave the town or kill the Rudder Master]
When the owner of the medicine shop explained the implication of Rule number twenty-one to Lu Yang and Meng Jingzhou, they were both reminded of the same idea.
Since they couldn't leave the town or kill the rudder master, which of them would have the upper hand if the two were put into competition?
If the answer is unknown, then they might as well try.
Either way, whether the Rudder Master leaves the town or if they kill him, it would be beneficial for them.
If the Rudder Master leaves the town, then Buyi town becomes theirs. They could find another way to leave, such as using the private school teacher as a leverage.
Moreover, both of them didn't believe that the Rudder Master leaving Buyi town wouldn't shake it up a bit.
If they could kill the Rudder Master, then that would be even better. The two of them would be able to surpass him directly.