It was all that, too, but it was also absolutely heartbreaking. I found myself in tears, but not just because it was sad - I felt an actual physical pain in my chest, because I so completely understood what she was feeling, and what she was thinking.I thought the atmosphere on the last ~5 minutes was utterly eerie and fragile.
But if you think about it, it was actually a happy ending. Moo-Hyuk had been abandoned all his life, and had never had love - even Eun-Chae abandoned him at one point. But in the end, she decided to die and be with him. She was completely accepting him, and loving him. It was the only thing she could do for him. I can just picture them now, together, happy...