Not really, I don't think he's that stupid considering he is able to pull of being blind to make more money, honestly he is pretty good at acting.
And as he is playing blind to most of the outside world, he can't exactly report is without some evidence.
He doesn't wanna investigate, he's just scared for his life and just wants proof.
Now he can't even do that cuz of his semen in the drain hole, they will think he is an accomplice.
If he had tried to leave that day, he sure would have been murdered, the chapters made that pretty obvious.
Because he knows it's pointless to run away from him. Given the fact that the killer has already done a background check on him(I feel he approached Ah-In after doing it) that's why he tests him right from the beginning. The killer feels this as a sort of a game which he plays whenever he's writing a new novel?
Because he's always there wherever Ah-In's present. He also knows Ah-In stole(I don't remember if the watch was the killer's). All in all he wants to see to what lengths Ah-In can play this before he kills him off.
Ah-In on the other hand feels it's just a one time thing and he can get away with it, with the first guy, then he continues it to gather evidence with the second guy, but now from his pov you can see how shocked he is that it got to his friend and bf. It's too late to even run away from him now

The protagonist is probably the most stupid I have seen in a while. He saw the dead body in the apartment but didn't leave, instead he followed Mr. Murder inside. He then managed to escape the place after finishing his "session"... Just to end up on the crime scene again the next day.. BY WILL. Why does he feel the need to "investigate" by himself and get himself killed? And why did he say "the lengths I take to live.." in chapter 11 when it's literally his own fault at this point..