말할 수 없는 비밀 (Unspeakable Secret) from Kill Me, Heal Me is quite sad. Here's a translation of the chorus:
Don’t love me
I can’t help my heart being like this
What I can give you
Are just unstoppable tears
With a strange face you’ve seen for the first time
I would only give you scars in your heart
I’m just going to leave you so
I can love you alone
Also, Moon Myung Jin's voice is incredible. You can really feel the emotion of the song.