i'm pretty sure the author confirmed that bum and sangwoo are both officially dead. don't quote me on that though
and if ur having trouble understanding the ending, a lot of people discussed it here and it might help you: https://www.reddit.com/r/KillingStalking/comments/b3lxoq/chapter_67_discussion/
SPOILERS!!!
so, as you know, Sangwoo most likely died because of the explosion and the old lady’s pillow. Yoonbum went to his house and accidently spilled his ashes. he got in a mental state where he believed Sangwoo was alive, and he started to chase that Sangwoo his mind created, but it wasn’t real, but Yoonbum thought it was. that imaginary Sangwoo was with a girl/woman and Yoonbum didn’t want to accept that. so Sangwoo and that woman was crossing the road, and Yoonbum went after them, but he did not realize that the lamp went red. it’s a bit foggy, but actually Seungbae hit Yoonbum, and Yoonbum died as well. this was an extremely sad ending for all the characters, because Seungbae killed the person he wanted to save in the first place.
i hope you can understand it a better now :))
I loved this anime although the ending did confuse me a little . the story line was amazing ! i loved little ole "bumbum" the characters lives were amazing as well ! truly thankful to the creators for blessing me with this anime . just wish it was still continued. many thanks :)