Yes I agree that Manik needs to accept that his goodness and his dark side can co-exist. He has to make peace with the demons of his past but at the same time embrace her light completely.
No human being is black or white...The reason he is upset could be his own insecurities. His inability to become the man she wants him to be, to have failed her, for her to have this confusion in her mind. He wanted to be good for her. But if she has doubts over him, then this transformation becomes irrelevant. Can he ever be the perfect man for her? Can she ever forget the past deeds of the monster? These thoughts are eating him up and hence this mask of indifference and hatred...