Why samar supported dimpy

Because he knows in this show all men r cheater n he has Mister Shah's genes like toshu n he too will cheat. 

So he is supporting dimpy so when he cheats her he can quote the same to her that other woman was just expressing her thanks to him n he just accepted the thanks.

This is the best explanation for his actions, I have read all day. Writers will definitely copy this idea whenever winter cheating track is shown and he'll speak this exact dialogue to justify it 

Are they going to make Dimpy a negative character later? 

No she will turn another Kinjal who will speak very high of women empowerment but will do opposite.

Dimpy is a rape victim so turning her negative  will look very wrong.

Hmm right. Makes sense. But they would soon need someone to create a mess.