A few questions (spoilers)

I re-watched this film recently and liked it very much. But there are a few things about the plot that I don't understand. Unfortunaetly I guess this is what happens when you have to rely on subtitles written in your second language (English in my case).

Essentially: what were they going to do? Did they think that they would get away with it? Was Kim going to be installed as president? Did they have any plan at all or were they just improvising? According to Wikipedia they were trying to make it look like a North Korean attack, but that's not something I understood even thought it was appearent that they tried to cover it up in some way by shooting the dead bodies.

And what was Yang's role? He first seemed to collaborate (moving the president's body to the hospital) but then tells the army man that Kim is responsible for the murder. Just seems confusing to me.

Thank you all!