So whoever kills a Muslim unlawfully is warned by Allah in His words: “And whoever kills a believer intentionally—his recompense is Hell, to abide therein forever; Allah is angry with him, has cursed him, and prepared for him a mighty punishment.”
The juristic ruling is that such a person is a fāsiq (an openly disobedient sinner), for he has perpetrated one of the gravest major sins. His fate rests with Allah alone: if He wills, He punishes him; and if He wills, He forgives him, as the Exalted says, “Indeed, Allah does not forgive that partners be associated with Him, but He forgives whatever is less than that for whomever He wills.” Thus the murderer remains under Allah’s will, because his sin is less than shirk.
This remains the case so long as he does not repent. If, however, he repents, his repentance is accepted, for Allah says: “Say, ‘O My servants who have transgressed against their own souls, do not despair of the mercy of Allah. Indeed, Allah forgives all sins; He is the All-Forgiving, the Merciful.’”
Yet mere repentance does not remove the victim’s right in the Hereafter. On the Day of Judgment the slain person will take from the murderer’s good deeds in proportion to the injustice done, or Allah will recompense him from His own bounty. Nor is the victim’s right annulled by the implementation of qiṣāṣ (retaliation), because qiṣāṣ belongs to the heirs of the murdered.
Imam Ibn al-Qayyim said: “The correct view is that killing involves three separate rights: the right of Allah, the right of the victim, and the right of the heir. If the killer surrenders himself voluntarily to the heir, out of remorse and fear of Allah, and repents a sincere repentance, then Allah’s right falls away by virtue of that repentance, and the heirs’ right is satisfied through retaliation, reconciliation, or pardon. What remains is the victim’s right, which Allah will compensate on the Day of Resurrection for His repentant servant, reconciling the two.”
According to the majority of scholars, homicide is divided into three categories: intentional killing, quasi-intentional killing, and accidental killing.