What's the deal with Sang-gon?

There is no explanation for the seperation of Joon Suk's father and Sang Gon. The only insight is at the restaurant when Sang Gon says "Some say I stuck a knife in the old bosses back." The guy in the car that Joon Suk runs after (the same old man that gets arrested in the sauna) is the newly crowned head of the family that Joon Suk's father headed. We can assume that the meeting of the two prior to this scene was some sort of formal transfer of power.

1. Jun-seok was forced into Sang-gon's presence because he used be a drug user/dealer under Sang-gon's authority. Sang-gon fed Jun-seok's habit either for free or at a discounted price. Sang-gon felt betrayed by Jun-seok's pledge of loyalty to the henchman's gang (which is Jun-seok's father's gang).

2. Sang-gon used to work under the henchman that visited Jun-seok's father. Sang-gon mutinied at some point against the Jun-seok's father to form his own gang. There are no details on how this happened other than it appears that Sang-gon initiated the coup in efforts to break the monopoly (or to bring "balance" in his own words).

3. The "other guy" is the henchman who worked under Jun-seok's father. He used to be Sang-gon's superior. He is now the head of the gang. Jun-seok pledged allegiance to him.