I don't think evilness needs an explanation. Bung Gu's clearly always had an inferiority complex, mixed with megalomania, a lethal combination which is found pretty often in deranged criminals, like mafia bosses and the like.
I actually like it that they didn't retort to some makjang explanation like a secret birth, and hope they won'e play that card in the last episodes.
When he confronts the Queen Mother, on episode 16, he starts explaining what a "pitiful" child he had been, with a vulgar mother who had to earn her living to send him to good schools (schools with royals, no less) and a father who possibly never recognized him until the end (which we see in the first episode: he hands the son Club M, something he clearly didn't want to do before, or the son would have killed him a lot sooner). And I truly loved the Queen's reply: "I have known many people who came from difficult households, but most of them haven't turned like you. They are good, honest people." In a word: don't blame parents, environment and chances to excuse your rotten nature.
Some people are simply evil, period. We can call them sick, but I believe this is offensive towards people with a real sickness.
So, on my part I am very glad this drama didn't go in such a predictable direction and chose to let the viewer be the judge of each character.
:)