I used to think that Dae-mok was smart, but his plan for maintaining power after murdering the royal family is...disappointing. And also accidentally hilarious. The other Seon does not look, sound, or act like the real Prince Seon. He couldn't fool Dae-mok for a single scene even when the unexpected help of a false witness. Yet apparently, the other Seon is now the lynchpin upon which all of Dae-mok's evil schemes rest.
The actual ambitions of Dae-mok's evil schemes are a tad inconsistent. He had a good thing going with the water price gouging scheme. Water is something everyone needs, but very few people know about the capital costs necessary to procure it. Rather than just continue with that, Dae-mok uses his victory to unnecessarily antagonize the peasantry. At which point Ga-eun comes up with an equally silly and improbable plan to escape, which of course works. Although this is mostly because Dae-mok straight up forgets that the peasants exist for five years.