[Pooya/Ali Commentary 2:179] Qassa, the root of qisas, means to cut, divide and differentiate. Life is balanced by discrimination between good and bad, light and darkness, life and death. The proper justice of retaliation is discrimination without which we become transgressors by not enforcing Allah's law of balance. Therefore, the guilty should be paid back in the same coin. The people against whom the crime is committed should know what punishment has been given and how it is carried out. This way the boundaries of transgression are clearly marked and made known. La-allakam tattaqun points out that the purpose of ordaining penalty for the crime is to make people abstain from evildoing.