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This is super simplified for anyone confused about the politics, basically the king's younger brother (official in purple garb heretofore known as Purple Man) who first comes to seek Heeryang out was concerned with the threat in the north and wanted to eliminate it, but was meeting opposition from politicians who were suspicious of his motivations. In order to secure the borders and do what he thought was best for the country, the Purple Man ordered Heeryang to eliminate the dissenting politician (MC's father if I'm not mistaken) in order to make the other politicians fall in line and agree to securing the borderlands.
After the campaign in the north succeeded and security was returned to the border, the border lords who are probably like feudal lords of fiefdoms a little like Japanese daimyos I imagine though maybe lesser ranked, being militaristic and survived on military campaigns and battles no longer had much purpose since there was not a great need for them. These border lords started raiding/pillaging the rural towns and basically started eating into their own nation's resources creating a new problem for Purple Man to try and solve. In order to squash this new problem Heeryang was employed to sow mistrust and fear among these lords so that they would start being suspicious of themselves and their compatriots turning their focus to in-fighting.
This is what I gather from the snippets shared. Feel free to correct any misunderstandings to help people get a clearer picture!