I've watched all of the released episodes and while I'm waiting, I am watching the first 7 again as it was confusing…
ML is not real Chang Ning. He is the young master of Demon Sect Mu Qing Yan. He was poisoned and heavily wounded by the current chief of Demon sect (Nie) and his followers and escaped to Chang's place where Chang Ning's dad saved him. (I think this happens a year ago before he meets FL) Chang Ning is someone else. (Jiang they showed before, the one who told that he is the illegitimate son of Chang elder) ML just assumed CN's identity for his own agenda, to take revenge. The one he talks about is real Chang Ning's father to Lingbo since he assumed CN's identity, he is playing CN's role there. He wants revenge for his Mu clan against Nies and take back his real position in demon sect and he also wants to take revenge for Chang elder (CN's dad) who saved him. Since Nie and followers are responsible for both the tragedies, basically he wants to clean up Demon sect which originally belonged to Mus and take back what is rightfully his. It seems someone from righteous sects are also in cahoots with Nies.