The monster than Hyun Soo killed actually didn't die, and it took over the gangster's dead body.
Eun Hyuk stayed behind because he was already infected and didn't want to trouble everyone by staying. I think there will likely be a second season so it's not the actual ending, just the season finale I'm hoping.
quite accurate portrayal of world religious cult like "illuminati"
Eh I see nothing like that from the first 2 episodes, it's just the police being extremely incompetent and stupid to the point that it's crazy to think they even work in that profession.