I'm pretty sure the bad guy is the one who was trying to accuse the MC and his father of stealing his serfs and was using that to try and strong arm them.
The MC basically fixed up his father's land, started a resort and the peasants from other lands were attracted to it, but this fat little dude got mad that his peasants left his land, so he tried to use the tsundere girl's father, who is the prime minister, to try and get compensation from the MC's father. But the tsundere girl who likes the MC convinced her father to side with the MC.