The way I see it every character in this story has an underlying need that they seek to fulfill (and which has been missing from early in their life) which motivates a good majority of their actions.
For NJ it is:
(a) complete authority
(b) pure love which also needs to comply with his need (a).
Both these needs arise from and unstable childhood and home. He grew up in a house where almost every positive reinforcement or affection was provided solely after/due to some sort of achievement. Growing up in such a household a person quickly comes to see success as the only means to complete happiness. Also what one lacks is the ability to deal with pure emotions. No, I'm not talking about how one feels happy after a good coffee or mad at a traffic jam. I'm talking about pure love and affection, jealousy and the other plethora of emotions that come when you form a connection with another person.
NJ has systematically compartmentalised his emotions to suit his cold world from a very early age. The only person towards whom he had pure emotions was a severely washed out and depressed mother who suffered from a litany of addictions. While she surely did love him, she herself was in no condition to be able to protect her child. When she k*lled herself she left behind a far worse trauma in NJ'S mind. It also birthed an unconscious fear that anyone he loves so entirely will leave him if doesn't make sure that he has all the strings in his hands.
At this point, I feel (rather sadistically) he needs to face this marrow deep trauma. He needs to feel out of control of the situation at hand (not necessarily with his rivals but even with Y/n) if only to understand that controlling everything doesn't equal to happiness. That people and their love can go beyond them being in his complete control.
It is such a blessing to be able to receive an analysis from you. You are phenomenal when it comes to reading between the lines and analyzing characters. You are right, Namjoon is yet to come to terms with his childhood trauma. It is bizarre how much one's childhood experiences are still underestimated when it comes to affecting one's emotional state and perception.
For Kim Namjoon, fulfilment comes from two sources-
Having a loving family- ideal parents, sweet, loved children and deep love between him and his partner.
And secondly, Power- Power and control over his life, over others, even his loved one(because he thinks that he knows what is the best for them, and that would also extend to his children).
But above all, everything had stemmed from his family and his odd relationship with his father. Namjoon wants to be far, far better than his father, not just professionally, but also personally. And this, infused with the burning need to fulfil the gap in his heart for pure, unadulterated love has turned into a smothering cage for (Y/N).
And even if Namjoon sees it, he does not want to admit it. He thinks that it is negligible and would only help both of them in the long run.
He also sees (Y/N) as his personal, most beloved possession. The sense of ownership makes him feel secure about their 'relationship'. So he also expects her submission and wants her trust(which he could never have completely)- but I would press more on the fact of trust because he loves to be the dependable provider, he believes that it gives him the upper hand.