DEMIGODS & SEMIDEVILS was extraordinary in the Jin Yong canon for many things, and one of them is that there was no real primary villain. There were evil people galore to be sure, but not a central nemesis such as LOCH's West Poison Au Yeung Fung, ROCH's Golden Wheel Monk, HSDS's Sing Kwun, SOD's Ngok But Kwun, etc.
If DGSD did have a "main villain," it might be Mo Yung F'uk. But can we really hold Mo Yung F'uk accountable for his actions? He had been brainwashed from childhood by his father to dedicate his life to the restoration of the Yin Kingdom. That was probably all he was taught (along with martial arts) during his childhood and adolescence, and it became the sole focus of his life. He literally knew nothing else. Every endeavor and experience in Mo Yung F'uk's life was just a step towards the ultimate goal of restoring the Yin Kingdom.
Mo Yung Bok re-emerged suddenly only to disappear again just as quickly into a life of meditation with the Janitor Monk. Mo Yung Bok had learned to give up the mad dream of restoring the Yin Kingdom, but left his son to carry on the dream that he had abdicated. Mo Yung Bok basically abandoned his son to deal with the consequences of the insane ambition he had stoked in the younger man. Ultimately, pursuit of this ambition drove Mo Yung F'uk insane.
Does Mo Yung F'uk bear the sole responsibility for what happened to him, or is he merely a victim of his father's schemes? Should he be condemned or pitied?