I am not sure I completely understand how things got so dark and changed to the extent they have in River Where the Moon Rises from the Crown Prince to General Go and even On Dal. It almost feels like everything the Princess and On Dal worked towards was for nothing. Not only does she lose a brother but a husband as well, not to death but something even worse the choices they made to better Goguryeo. The good guys won, but that win came at such a high price that not even love could bear the cost. The show does a fantastic job at showing exactly that— at times, winning isn't everything.
I completely understood On Dal's emotional breakdown; everything finally came to a head with his mother's murder, the wars, the bloodshed; it was all too much to bear. I felt for him deeply. To lose the one person that tried the hardest to protect him, I'm must be unbearable but to decide and leave everything behind to live a hermit's life with delusions of his dead mother to make up for that loss seemed extreme, but then everybody grief in different ways. I know he said he decided to leave the Princess for her sake, but I am not entirely sure that is the whole truth. It's almost like he is running away from the person he let himself become. Their reunion in the mountain after four years apart was as sweet as it was sad.
What starts in greed, malice, deceit, lewdness, envy, slander, arrogance, and folly was always bound to end in the worst of ways. To put it simply, Go Pyo Won and General Go's evildoings finally caught up to them. Rather than accepting their just punishment, both father and son shamelessly continued to believe they had every right to their treachery, especially General Go. When I first started this drama, I was captivated by him, but he became the biggest disappointment. He went from a man with principle to one trapped in obsession, jealousy, and greed to a traitor.
The other biggest disappointments are the Crown Prince (now King) and Mo Young. I get Mo Young decided to follow her heart and chose a man she knew was obsessed with Pyeonggang and decided to believe he loved her. I doubt General Go ever knew true love or how to love. General Go blaming Pyeonggang for everything while still pining, for she spoke of how cunning, obsessive, and dangerous the General became. I was not surprised by this behavior, the King I don't get.
What happened to change him from a young Crown Prince with the potential to be a great ruler to the King he's become — selfish, arrogant, jealous of his sibling, and very one dimensional. How and why he let himself become this person is quite perplexing. I'm not sure if any of the characters can come back from the lines they crossed. As they say, some lines once crossed can never be uncrossed, but if you are going to cross that line, you might as well go all the way.