I think that one of the most basic themes of the Tagore short story is the reality of abandonment. Tagore plays with this theme quite a bit. The final scene between Kiran and Nilkantha is one where each experiences their own form of abandonment. On one hand, she abandons her initial belief in Nilkantha's innocence. When she sees the ink stand amongst his things, she is convinced that he is a thief. The abandonment of her initial feelings towards him and her unfettered love for him becomes a significant part of her characterization.
For Nilkantha, his own experience of emotional abandonment is complete at this moment. As the narrative has advanced, he had been feeling estranged from her with the presence of her brother. The competition for love with such an embedded part of her life as opposed to him essentially being an orphan had taken its toll. This became evident when he sees Kiran's eyes look at him as if he is a thief. His abandonment becomes clear at this moment as Tagore constructs his loneliness with an inability to speak:
He had seen the whole thing and thought that Kiran had come like a thief to catch him in his thieving, and that his deed was out. How could he ever hope to convince her that he was not a thief, and that only revenge had prompted him to take the inkstand, which he meant to throw into the river at the first chance? In a weak moment he had put it in the box instead. “He was not a thief,” his heart cried out, “not a thief!” Then what was he? What could he say? That he had stolen, and yet he was not a thief? He could never explain to Kiran how grievously wrong she was. And then, how could he bear the thought that she had tried to spy on him?
Nilkantha knows that he is "not a thief." Yet, he is trapped with this perception of reality and his own condition. It is here in which the theme of abandonment is most evident. He leaves her and walks away from everything because of such a condition. Almost as if to reiterate the theme, Nilkantha abandons the dog that he befriended, seeming to complete the replication of abandonment. His own abandonment of the dog is reflection of what he sees as Kiran's abandonment of him, as the dog "remained prowling along the river-bank, whining and whining as if its heart would break."