I am not defending him but I have an explanation for this..
He already told her that hero, despite being loved by all, is in love with heroine.. His heart says Madhu, Madhu instead of lubb, dubb... He actually told her that he loves her. How he was not able to stop thinking about her after revenge. ( he knew what he was talking about at that point, the craziness started once he started talking about Madhu's questionable feelings toward Sultan)
And then he was explaining to her that at that time when he saw bhai n Madhu together, she thought RK didnt love her, but she was still in love with him regardless he loved her or not.. I don't think it meant that he doesn't love her.. This is my take.. Tell me if I am wrong..? He mentioned earlier that only he has right to love her..