Maybe Muncheon has been working behind the scenes for centuries to make the martial sects grow rotten from the inside. In the end Muncheon has deep knowledge of martial arts, probably also plenty of forbidden arts, so they could help, in secret, corrupt individuals in other sects grow strong and gain significant positions. Since strength is everything in the murim, if a person isn't openly and overly evil in a righteous sect, they could advance in the ranks and have a poor influence on everyone afterwards, making new generations arrogant, lazy, and ruthless.