"Sung Mo, well he's been shot and is now mentally disabled. And, yes, MW played a role. (I think.)"

Minwoo went to get the tape, but Sangmo told him to give it back - as you already saw. The whole thing in that scene was that MW was just in disbelief - its like every person the guy ever loved turned out to be a betrayer or someone who just wanted to use him to their advantage. So MW was being simply very emotional in that scene.

Then when he heard the gun shot however, he ran out to see what was going on. And the first thing he asked the right hand man was "What happened to Sangmo Hyung?" -- in a frantic prose -- he was worried, lost and just a bit insane that moment. So he was worried about Sangmo.

MW really didn't play much part in Sangmo going missing. All Minwoo did in that scene was cry. And Sangmo already got his stuff and was on his way out -- and MW didn't even make the attempt to hold him back. It was all that right hand man's fault.

While watching it live, I had a tidbit hope that maybe MW would come after Sangmo, see his wounded condition, and then take him out of the car and then find the girl colleague and ask her to take Sangmo somewhere safe. Or he takes the colleague and Sangmo hyung and puts them somewhere safe while supplying them with enough money -- in a way taking care of them.

Hey I think that could of worked and in way make a good development of MW. But the writer of Giant of course is better at storytelling.