Before I answer this question, I think it is important to understand 19th Century cultural sentiments of the Bengali working class.
Bengal is a state in Eastern India, on the coast of the Bay of Bengal. Climatically, it is hot and humid, with an abundance of rain fall during the monsoon months of July, August and September. This type of climate gives rise to luscious greenery on the one hand, but also the heat and humidity tend to make its inhabitants lethargic. Add to that the typically Hindu philosophical attitude of "niyati," fate, and the cultural stance of the average Hindu poor is resignation -- to the rich and mighty, to God.
Rabindranath was extremely critical of this laid back attitude; he was also extremely disapproving of the ways of the landed rich (his own father was one) and he often wrote interesting short stories that pit such cultural attitudes like submission to fate against the need to take action.
In the light of the above, "Punishment" is a heart-rending story about ignorance, superstition and cultural biases against women on the one hand, and, on the other, one woman's defiance against all this.
Chandara sacrifices her own life after being falsely accused of murdering her sister-in-law because she WANTED her husband -- the false accuser -- to suffer guilt for the rest of his life. Her husband's brother, Dukhiram, had killed his own wife. His brother, Chidam, Chandara's husband, had witnessed it. Yet, when the authorities came, Chidam lied to them, accusing his wife of the murder.
On face-value, his reasoning is simple; simplistic, in fact: males are to be treasured; females are dispensible; you can marry another wife, but you cannot get another brothe But, lurking beneath inane belief-system, is a series of superstition that ate away at the Bengali culture of the working class poor. A woman's love is corrupted by her sex: her genitals, humid as the Bengali climate, are capable of receiving any man, if he so wishes to possess her. Weak as she is, she is vulnerable, and with her, the family into which she is married is made vulnerable! Tagore does not directly state any of this. It comes through with his brilliant use of inuendoes and metaphors.
Tagore could have ended the story here, and it would have been a blatant criticism of ignorant village life. But Rabindranath was nothing if he was not a deep and astute reader of human frailty and human strength. He infuses Chandara's story with indirect metaphorical references to sharp, breathtaking passionate love between Chandara and her husband. There are many, many subtle suggestions of Chidam's sexual addicttion for Chandara. Tagore suggests that it is his own strong sexual addiction that Chandara's husband feared the most: if she can give in to him so passionately, with such complete abundance, who knows whether she will succumb to another man's strength!
The accusation of murder thus comes almost unplanned -- suddeenly -- but was it something he had always contemplated? Subconsciously?
Who is being punished? Not Chandara. In my opinion, it is her husband who brings punishment down upon himself. He loses the only woman he loved; and Chandara, by embracing death and REFUSING to see her husband again, casts him away for ever to be tormented by the fury of his own hellish guilt!