«فَإِنْ كَانَ عِقَابُكَ إِيَّاهُمْ بِقَدْرِ ذُنُوبِهِمْ، كَانَ كَفَافًا، لَا لَكَ وَلَا عَلَيْكَ».
Translation: If the punishment matched their wrongdoing, the account would be even: neither a surplus in his favor nor a liability against him.
Fa-in introduces a condition, bi-qadri indicates measure or proportion, and kafāfan conveys equivalence.
This is not permission to calculate alleged wrongs privately and impose punishments. The narration teaches exactness in Allah's reckoning. In daily life, any response must still meet lawful standards of authority, due process and others' rights.