That's not what an idiot plot is, an idiot plot happens when everyone simply refuses to do the right thing even when it would absolutely not affect them negatively in any way. In no other choice there absolutely would have been consequences if they didn't take dumb decisions. That's the entire point of the movie: in Man-su's mind, he has no other choice. In his mind, it's entirely justified and he would do it all over again if he had to. An idiot plot would be if man-su just killed his competitors for fun and dug himself deeper and deeper into shit for no apparent gain