I think he came from a world, Adreda was it?, where selfishness and immortality rule best. I suppose he found new meaning in life when he met Yoon.. he is still erratic but he does love Yoon to the core (▰˘◡˘▰)
It wasn’t clarified how but he was born in the same world as Yoon. But he knew right from the start he was different. And he was cultivating in his own way. When Yoon ‘killed’ him in a fit of anger at the start his soul was sent to the other world/dimension - Andreda. And Yoon regressed into Mo Ran body at a younger time probably around the time he started cultivating. Because Yoon unintentionally interrupted Mo Rans breaking through a peak and the backsplash deteriorated his health but also somehow connected them. Mo Ran retuned into his own body after the incident of Yoon’ killing Mo Ran was repeated. But for Mo Ran a lot of time has passed because of the spend of time is different in both worlds. It’s still a mystery who was in Yoon’s own body but they acted just like him.
That’s why his actions are hard to discern. He is waaaay older than his body. The only time he didn’t care was when it was uncertain whether he wants to full out commit himself to Yoon knowing Yoon would die young (this was after Yoon confessed) otherwise he has always shown his interest and even possessiveness over Yoon.
You are right about the other world’s survival of the fittest and only care for yourself environment.
I couldn't quite understand Moran sometimes. It feels like he doesn't care but at times it felt like he does. At one time he would sacrifice himself and would do something the next. (╯°Д °)╯╧╧