He is thinking that he is saving Sitara from Qasim. The situation Sitara is in, it's difficult for her because no one in her family cares for her. Qasim is a grey character. May be, if she gets married to him and shifts with him to Islamabad her life will be a bit better compared to her life here.
At present, her situation is difficult because her uncle also turned his back. It is better if she goes to a place where she can have some peace. Like she said in one of the initial episodes that she wants to stay in a small house where she has the choice to choose the plants or something.
If she gets married to Salar her life won't be much different. She will not get the acceptance from his father and gets trapped in the plotting. Only and only if he is so much in love with her and can't live without her(I know it's a bit filmy but still....) and does so much to get married to her then I would be ok with their marriage.
If Sitara and Salaar's marriage happens, then I fear he will divorce her at some point, and then she'll again go back to Qasim, which I don't want 😢
If that doesn't happen, then I think Qasim will try to break their married life, but they will clear all misunderstandings and get a happy ending in the end.
So he thinks that he is the victim now , and for this he is going to stop Sitara and Qasim's marriage!
I thought he was a better human to do so, specially after his sister explained to him how a middle class girl's dignity can be trashed for a mere doubt. 
I think one good day Nimi's bhabi will tell Sitara what she heared from Kiran and Nimi's discussion, and that Qasim knew that Nimi sent those messages to Sitara . 
