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I love the ending! I really enjoyed the growth of the main from the beginning all the way until the end. He finally got the girl and a happy ending T_T
If you still don't understand the ending, I can summarize it for you.
It turns out that Moon Bin was taken to the past by the murdered sister of Sa Kyoung to avenge her and her murdered family. The real Sa Kyoung actually died right before Moon Bin's time travel trip, so he inhabited the body of Sa Kyoung. All the while, Moon Bin never understood why the sister haunted him rather than her murderer, Sali Tayi. As Moon Bin (aka Sa Kyoung) plotted to get revenge on Sali Tayi, he realized that the sister was haunting her murderer all along! Because Moon Bin was Sali Tayi in a past life! When Moon Bin killed Sali Tayi and allowed himself to "die",he finally broke the chain of karma by ending the cycle of hatred and avenging Sa Kyoung's family.
When he woke up, he learned from his experience and began to appreciate everything that was lost to him in the past... ie. his parents (everything that Sali Tayi lacked) and chose to abandon the way of the sword... as a symbolic way to shed his past. As the monk said... who you were in the past doesn't matter... you can choose you are and who you become. When he did that, his life finally seemed to come together and he finally got the girl he was waiting for through the ages (the girl that he couldn't get in his past life as Sali Tayi).