What is the required شرعي (legally prescribed) action to take towards a clan that treacherously killed a Muslim after prayer, then sniped and killed another one of his relatives, while the killers are free and the authorities have not prosecuted them?

The soul of a believer is mighty in the sight of Allah. Intentionally killing a believer is recompensed with Hell, Allah's wrath, His curse, and a tremendous torment. The first matter to be judged on the Day of Resurrection will be bloodshed. The perishing of the world is less significant to Allah than the unjust killing of a believer. Therefore, whoever kills a believer unjustly must repent before the opportunity is lost.

Those who conspire to kill are all to be executed, unless the victim's guardians pardon them or accept blood money. Umar ibn al-Khattab, may Allah be pleased with him, said concerning a man upon whom a group conspired: "Had the people of Sana' conspired against him, I would have killed them all." The law of Islam does not permit an individual to seek retribution on their own; rather, retribution (qisas) and punishments (hudud) are the prerogative of the ruling authority.