Josh is one of those people that if you met him in the real world, you'd probably never know he was like this.
He's very intelligent but extremely lacking in the social department. Josh genuinely thinks that Nayeon is aware of him and loves him back and that he's earned the right to be with her because he "loves" her and has spent an assload of money to see her. And because of that, he thinks he has the right to kill her if she "betrays" him by dating or just doesn't love him back. The man has actual reasoning behind everything he does and it's pretty solid, aside from the murder bit
I don't think his parents have taken him seriously even though they know what he's doing. It'd be better if he was a twitchy lunatic talking to the walls, then everyone would see it and have him committed or medicated