There used to be a pdf file outlining all plays based on this story in the Mahabharata, which I cannot find anymore. However, I am familiar with the version referenced by the movie, which I reproduce here.
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Duryodhana orders a tantrist Malayan to kill Pandavas by black magic. Malayan refuses
saying that both Pandavas and Kauravas are kings to him. Duryodhana threatens to kill Malayan if he doesnt obey.

Malayan tries to stall by demanding all sorts of impossible things. He demands that Dussala (Duryodhana’s sister) should be sacrificed for the ritual, enraging Duryodhana, who refuses to give him anything, and repeats the threat to kill him.

Malayan goes ahead and kills the Pandavas by stabbing their shadows (which is the literal translation of nizhalkkuthu).

Malayan is then praised by Duryodhana who
showers him with gifts. But, Malayan is very despondent since he believes he has sinned because of his choice to kill the Pandavas, even though he had little choice in the matter.

Seeing him unhappy, his wife asks the reason.
Malayan tells her that on the way killed 5 deer by nizhalkkutthu. The worried wife further questions him, and Malayan admits that he was forced by Duryodhana to kill the Pandavas by nizhalkkuttu.

Hearing this, the wife is devastated and enraged. She tells him that she would teach him about Kunti’s (the mother of Pandavas) grief—and kills her own son.

The reason, imo, is in the story the film is named after. The story comes from the Mahabharata and it is about the futility of some punishments.

Without going into too much detail, in exact parallel to the Mahabharata story, the executioner here (the son) believes that he is making the father feel the remorse/sin/shame that should have accompanied prior executions, and thus punishing him. All the while being blind to his own crime, and blind to the fact that the father is already devastated.

The film is ultimately a warning that we may be just as blind when it comes to punishments in general, and death penalty in particular. What we believe is just may be completely ridiculous.

It is interesting to me that you thought it had something to do with tradition or fate. It sort of illustrates to me why there is a huge communication gap even with thoughtful people in the west and half the English-only speaking crowd in India—like here, maybe there is always a “background story” missing?