Is it permissible for one of the slain's brothers to intentionally kill the murderer before reconciliation, and is it permissible to kill him after the reconciliation that the family was coerced into by a committee in the city?

It is not permissible for the slain person's brother to take revenge on the killer himself. Rather, he should refer the matter to the authorities. If he kills him, he is sinful, because retribution (qisas) is carried out by those in authority. It is not permissible to kill the killer unless the guardians of the slain person agree upon retribution. If one of the heirs pardons retribution, the right to retribution is forfeited, and the remaining heirs are entitled to their share of the blood money (diyah).