Here is the issue.
Kunal does not think Siddhi is character less. He was just so much affected by the way Abhay presented matter. Abhay saying thank God her ex left so now she's mine. The picture appeard as if Siddhi too wanted that while she has no idea about it. Abhay saying he believes Siddhi too likes him but not saying it due to her ex but they live in the same house.
What really went bad with him was abhay taking siddhi away from him saying he will take care of her. There though uncomfortable, siddhi moved with him. Kunal puts that with lie about siddhi being gold-digger says they are in relationship.
But believe it or not, kunal struggled to accept that & internal turmoil resulted in venting off through harsh words. If he really believes that, no need to call Siddhi on that matter. He would easily accept it & let go. But time after time he tries to protect her dignity as he just can't accept anyone questioning Siddhi.
Simple matter is kunal himself is trying very hard to convince himself of what is presented ad true but heart refuses & such impuldive behavior.
They are hubby-wife so they can be rude to each other. When Siddhi saw richa wiping cake off Kunal she thought kunal betrayed her as he's married & didn't care about their son either. Not going to blame her either.
Other people, sad but reality guys do look in thid manner to any woman.
The matter in article is for Siddhi to see while kunal hurt her baldly, he himself can't stand anyone pointing fingers at her so she would think what urged kunal to say what he did.