Promises are really meant to be fulfilled. Yoonbum followed Sangwoo to death as promised.

I know! This was one of the most important pieces of the ending but hardly anybody pointed it out

Indeed, i also remembered Sangwoo's mom tried to kill him with a pillow when he was a child. How coincidentally and ironically he did die from the same assault at the end. Like how more tragic the ending can be dksksksksk