What is the ruling on exacting retribution (qisas) from a premeditated murderer in the absence of a Muslim ruler, and does the principle of killing a group for an individual apply to an assassination (qatl ghilah) in which more than eight people participated?

The establishment and execution of the hudud (Islamic penal laws) are entrusted to the ruler (wali al-amr) or his representative, due to the order and prevention of corruption and chaos that result from it. As for assassination (qatl al-ghila), there is no pardon for it, and all those who participated in it are to be killed, as Umar ibn al-Khattab, may Allah be pleased with him, did when he killed five or seven men for a single man they had assassinated, and his saying: "If the people of Sana'a had conspired against him, I would have killed them all." And as the Prophet, peace and blessings be upon him, killed Al-Harith ibn Suwayd for Al-Mujadhdhar ibn Dhiyad by assassination. However, the execution of this judgment is for the ruler only.