Kunal is following his belief system

Kunal had this resigned, exhausted, defeated look about his body language today. His patience and tolerance for everything that is happening around him has clearly reached a saturation point.

It doesn't help that his family members keep exhibiting a certain bias which only makes him feel further more ignored and alienated.

Kunal's ways of going about things might not be morally upright always but in all honesty deep down he is just this decent guy who grew up to believe certain notions and tries his best to live upto those as he doesn't know any other way of being.

Just like how Abir is blinded to believe that his father is a great man (when actually he is not) Kunal grew up with a belief system that family always comes first and if ever he is to make a choice between his brother and lover, he would always only choose his brother (in a heartbeat, without any second thoughts) - we can debate and say that he doesn't love Kuhu just yet but Kuhu is definitely one person he was growing to count on. On one instance he did not simply seek her suggestion but also followed it and thanked her for the same too. If not love, this was one relationship that meant *something* to him but clearly it's far from being SO important that he would let it get precedence over something that he deems right for his brother & family - that is Mishti going away from them. Hence the idea of divorce resonated so strongly for him as that would serve as the way through which MishBir's relationship would get destroyed.

Here I would also like to add that Kunal's hurt feelings and repressed anger that he has towards his mother and brother actually come out in the form of vitriolic that he ends up directing at Mishti and Kuhu.

Meenakshi is greatly responsible for Kunal's exaggerated hatred towards Mishti because she made him party to a plan of ruining a girl's life in the guise of separating Mishti from Abir.

And Abir in that regard has caused more hurt simply by showing ignorance. Even today he constantly kept saying that "tu meri khushi mein khush nahin hoga" something which I thought was so banal and dumb. How is Kunal expected to be happy for Abir when clearly he doesn't like Mishti? Would Abir have been happy if Kunal's happiness lied in getting married to Shweta? Not that am drawing any parallels between Mishti and Shweta, no. But clearly Abir was not even trying to address the crux of the problem and was simply talking to Kunal in a superficial manner.

Anyhoo, back to the topic, I feel Kunal has this signature business like way of going about problems. He's not equipped with the tools of treading with relationships in a nuanced way so keeps tackling problems in a way he feels he knows best and succeeds at - like a business deal. He also wants to stay as detached from emotions as possible because of the experiences he has had so we don't see him so much as bat an eye when he calls Mr. Kedia to get the divorce papers ready as early as possible, when actually just hours back he was being so genuinely humble and nice with Kuhu (while asking her for the timeout)