[spoiler warning for anyone who doesn't want to be spoiled]
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in the end, sangwoo got hospitalized from the explosion when he fought the cop, seungbae. eventually bum decides to visit sangwoo in the hospital so he can give him a ring, but instead, he gets informed that sangwoo had already died 2 days earlier because an old lady heard sangwoo calling bum's name out every night in the hospital bed and she thought it was annoying so she suffocated him with a pillow. so bum gets depressed, and he starts having hallucinations of sangwoo and begins following his hallucination, until the story ends with him following it across the crosswalk while the light is red. a lot of people think bum died, and that basically fulfills sangwoo and bum's promise: dying together (i don't remember what chapter it was, but bum promised to sangwoo that he will die when he dies). i hope this answered ur question

Im crying so hard right now i stayed up until 4am to finish the last chapters that i didnt get to read TT
But i guess this is the best ending for thembc they were so unstable afterall... im just sad... i think i'll be in a corner blankly staring at the wall and rethinking my life choices now....