If you have watched the whole series you should have understood that Duankang had been through so much so he did it in his defense. He had been tricked, scammed, lied, and beat up hundred of times. The most important thing in his life was Ruyi, more than his family. He could have lived and died just for her and i think Ruyi did the same for him. She left the royal and ran away with him. She married a bastard to save his life. She pretended to be crazy so shichang would leave duankang alone and for his safety. She was injured but decided to leave the mountain so she won't be a burden for him. She came back for him but again prentended to be somebody else so he could married xiabao. She had nobody, no family. Her whole life was a tragedy because she loved and chose him. But I'm glad there was a happy ending for her, for them.