What is the punishment for someone who killed a person who insulted God, Islam, and the Messenger, may God's prayers and peace be upon him, and declared his disbelief after having been a Muslim, given that the killer could not control himself, and knowing that he will be executed in this world as a retribution (qisas)?

The state must rule by the Sharia of Allah, and the implementation of hudud (punishments) is the prerogative of the Imam or his deputy. It is not permissible for anyone else to apply them except by delegation. "When the hadd for adultery, theft, or drinking becomes obligatory, it is not permissible to carry it out except by order of the Imam, or by order of someone authorized by the Imam." And "there is no disagreement that qisas (retribution) in murder is not carried out except by those in authority." As for one who initiates the killing of an apostate, he is not killed in return as long as the slain person's blood is not protected. Rather, he is to be disciplined (ta'zeer) for overstepping the authority of the ruler, as stated in Al-Furu' by Ibn Muflih: "Whoever kills an apostate, or a married fornicator... his blood is in vain, and he is to be disciplined for overstepping the authority of the ruler." As for the Hereafter, there is no text regarding his punishment, nor is he considered as one who killed a protected soul.