Why? What is the motive behind Jun Young's actions!

*Spoilers Ahead*

Just like everyone watching I wanted to understand the motive or intentions of the antagonist too. After the second watch, it just feels like he wanted sense of control and power just like every other psychopath or sociopath. With help of technology and the amount of dependency, victims have on technology, he was able to understand even the tiniest thing. The likes, dislikes, secrets, friends, foes, etc.

He played and manipulated victims, just to exert the power of how he can completely control their lives. Isolating them from loved ones, manipulating them into trusting only him, and in the end killing them once the control game has satisfied them. As he tells her dad, as long as she does not get suspicious of me like you, she will be safe for a while. 

It is a fun play for him, a Semi God like feeling. He thinks of his victims as utter fools. He sees their life and misery as an act of idiocy, he thinks it's stupid that victims are first being completely dependent on a device/tech and second being irresponsible enough to lose it.

I don't know if being an orphan has some role in it. But more often than not, psychopaths and sociopaths have no solid reason to act the way do. It's just the sense of fun and fulfilment they get out of it.

What do you think? Let me know your thoughts. 

he hallucinates a dentist 

At the end of the film media say that he was a person born but not existing in the list of existing people, in short, a person without a name.
When the policeman took his notes, there he says he wants to become a serial killer by taking the name of his deceased son and blame him.

Not all psychopaths have or need reasons or motives to kill......in other psychopath movies the killer are given a background as they have a troubled childhood or trauma but here the killer is anonymous so the background part is missing and the only reason he stated was that the victim lost their phones and he found them. few days ago i watched another psychological thriller "We Need to Talk About Kevin" kind of same as this one the killer didnt have a reason for the crimes he did and didnt particularly had a troubled past .