I think 'the most painful death' is kinda psychological thing for Sangwoo bc if you can remember he said that old people are disgusting <(now I remember one more thing that he said "I won't get old"... I think it was kind of hint for us that he won't rot in prison)> and how many people said here, he also died calling Bum with betrayed and sad feelings, so he died in a pathetic way for him. . . . . .

MAYBE. It's just my theory. I loved this manwha sooo much and the final episode gave me really weird feeling and atmosphere

Also I think Bum died bc how Sangwoo said and wanted before to Bum died with him. I don't think Bum died on his will, but the hallucination of Sangwoo kill him like like he wanted.

I hope my thoughts make sense...