Nobody was screaming or cowering in fear, so I decided not to run, but to ask him what he was doing. Turns out he's a magpie hunter. The dastardly magpies of Itaewon are nesting in the pylons and so must be shot.
The writing on his jacket is 유해 야생 동물 (까치) 기동 포획 반 (Which I think means "harmful wild animal (magpie) rapid capture group").
I saw him take one down, but when I ventured outside later on there were still plenty of magpies and no sign of the hunter. I wonder if I'll see him and his rifle in these parts again.
(HDR photos processed with photomatix)