Came out of nowhere. Made no sense. Clearly done for a shock factor. So stupid. This shit ...

I get being upset but "came out of nowhere"??
They hinted at him dying all throughout the story. I think the way he died is stupid tho because why would jaeshin leave his side after being on alert

I think that was the point. You can’t be on your guard forever. To the both of them, all of the major threats to his life have been eradicated. I just wish it clicked immediately that they could probably be there for Nok (unless that’s what you meant as well)

I think the way he died made so much sense. They've been through the fire, protected him from every major threat time and time again. Only for a little old man Jaeshin doesn't even remember to give the final blow. No one saw that coming. I'm pretty sure this will eat Jae alive forever. His past wrongdoings have come to collect, in the most ridiculous way. I think that's the point the author is trying to make.

But also that old man gave us a hint the first time Nok and Jae went out.

!!!

I think that would have worked more if jaeshin's guard was down and the old man came out of nowhere. However, the attack came after jaeshin was on guard again. He sensed something was off.
Jaeshin is written as super protective of nok. I don't get why he'd leave him alone after feeling like something was off.
All throughout the story, whenever he sensed danger, he stayed with nok or left him protected.
But to leave him exposed and alone in an alley?? It's super out of character imo.