Who kills the boss, Seok, at the start of 'New World'?

It's never explained in the film, but I would suspect there are two possibilities:

1. Section Chief Kang
   It's possible he chose to do it, or orchestrate it, as part of the takedown of GoldMoon. After the leader is taken down, the entire organisation is thrown into chaos in a power struggle and they are able to get a real foothold of power in the gang.

2. Vice Chairman Jang Su-Ki
   He wanted power and knew he had a chance to get it if the boss was killed. He could have acted on his own initiative or been lured into doing it by the Section Chief. Note - the other candidates for the leadership appear to accuse/suspect each other which is why I don't think they can be considered responsible.

Ultimately though, there's no right answer - just speculation!

I just re-watched this movie and on second viewing I believe the director does reveal who killed the don.

When Chief Kang stops Jung in the airport he makes a comment that Jung was eliminating the don's guys by accusing them of being moles (ie. the opening scene where a "rat" is killed), and that the next step was killing the don so he could get the throne.

Just watch this scene, the way Chief Kang accuses him he's dead serious. And the way Jung responds, the director first does a shot of Jung being speechless, then after Kang says "Right?", Jung finally brushes it off.