Rasul Allah (sal Allahu alaihi wa sallam) said: “The blood of a Muslim man who testifies that there is no god but Allah and that I am the Messenger of Allah is not lawful to shed unless he be one of three: a married adulterer, someone killed in retaliation for killing another, or someone who abandons his religion and the Muslim community.” [Muzani in Al-Mukhtasar]
Killing without right is, after unbelief, one of the very worst enormities (Kaba’ir – enormous sins). This hadith delineates the only three cases when a Muslim’s life may be taken.
One is to put an end to adultery in such a way that it also deters others from approaching it. Adultery destroys families. The importance of the family in Allah’s sight is such that to protect it He has set a severe punishment for those who ruin it.
Secondly, a Muslim’s life may be taken in retaliation for his taking another’s life. When murderers are not brought to justice it devalues life, diminishes the enormity of the crime, and promotes the killing of innocent people.
Finally, a Muslim’s life may be taken if he apostasizes from Islam, becoming an unbeliever (Kafir). “Among the things that entail apostasy from Islam (may Allah protect us from them) are:
(1) to prostrate to an idol;
(2) to intend to commit unbelief;
(3) to speak words that imply unbelief such as, ‘Allah is the third of three,’ or ‘I am Allah’;
(4) to revile Allah or His messenger (sal Allahu alaihi wa sallam);
(5) to deny the existence of Allah, His beginningless and endless eternality;
(6) to be sarcastic about Allah’s name, command, promise or threat;
(7) to deny any verse of the Quran or add anything to it;
(8) to deny the obligatoriness of the prayer, zakat, fasting Ramadhan, Hajj, or the unlawfulness of wine and adultery, or anything else which is necessarily known as being of the religion;
(9) to say such things as, ‘I act without the decree of Allah’;
(10) to say that a Muslilm is an unbeliever;
(11) to refuse to teach a person the testification of faith (Shahada) when he asks to be taught it;
(12) to hold that any of Allah’s messengers or prophets are liars, or to deny their being sent;
(13) to revile the religion of Islam;
(14) to believe that things in themselves or by their own nature have any causal influence independent of the will of Allah;
(15) to deny the existence of angels or jinn;
(16) to be sarcastic about any ruling of the Shariah;
(17) to deny that Allah intended the Prophet’s message to be the religion followed by the entire world.” [Reliance of the Traveller by Ahmad ibn Naqib al-Misri]