Hmmm. I see where you are coming from. But ever since the revenge thingy jumped into the script , I have given a permanent leave to sense of logic and sensibility of all the three brothers.
Sameer does appear a stone-hearted devil - he is , for the moment, he only remembers his father's tragedy, his mother's terrible mental and physical state and the tears of his brothers. In front of that massive personal tragedy , all his earlier consideration for Avni has gone for a toss - he is no longer the Sam who fought with men at a pub because they dared to humiliate her .He is no longer the Sameer who wanted to make Avni see the world and herself in a new light.....for now , it is only raw vengeful emotion playing out.
There is no justification for what is happening but then... when people get hurt... they cause hurt. You can only hope sense to prevail upon all.