i think the whole reason he is leaving is kinda explained with the last few pages of this chapter.
Ijuuin was trying to convince him to stay but turns away to run back once he sees a patient in need (ambulance drove by). Asada sees lots of patients that needs saving with NGO, hence later on you see he is back in war zone area.
In the end, Asada just want as much patients saved as possible, he knows Mejin got it covered with his whole crew there.
Thanks for the picking this series back up, i never got to finish reading the ending when i first read this manga.