Hands down, this cracks my top 3 manhwa of all time. I don't really think of it as yaoi, I see it as a very well constructed psychological thriller because it's about more than just Sangwoo and Bum's "romantic" AKA abusive relationship. It actually puts you in the mindset of the characters, it is so well thought out, and is told in a stream of consciousness manner to keep you on your toes. Absolutely AMAZING. I cannot talk highly enough about this one, Koogi is on a whole other level with storytelling and art. Just READ IT.
Killing Stalking