1- If a man entered his home and saw his wife sleeping with a stranger and he killed them both, what will be Islam's judgment on him? Should he be executed too or what?2- If a Muslim killed another Muslim for no reason, will he be considered out of Islam (murtad)?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

If any man who finds his wife having sex with another man, kills that man or his wife, or both of them, he is killed for that as a legal retribution for his act. Merely saying that he found the man he killed committing Zina (fornication/adultery) with his wife does not influence the retribution verdict. The reason for this is that a man might want to kill another, so he urges that man by different means to come to his house and kill him and claim that the man was having sex with his wife. Islam has precautionary measures to protect life and avoid dissolute acts. ‘Ali was asked about a man who kills another man he found with his wife, and he said: “If he does not have four witnesses (on the act of adultery) tie him and deliver him to the authorities.” The authorities could decide on his fate. But if he has a sound evidence proving his claim, no retribution is applied on him and he is not asked to pay anything, for the following evidence: It is narrated from sa’eed Ibn Mansour that he reported from Caliph ‘Umar that a man came running to him holding a sword covered by blood and followed by some people who wanted to catch him. When he sat before ‘Umar the people who were following him came and said: “O Caliph of Muslims! This man killed our friend.” ‘Umar said to the man: “What is your reply?” The man said: “I struck between the thighs of my wife and if there were somebody there I have killed him.” ‘Umar said to the people who were following the man: “And what do you say? “Oh! Caliph of Muslims, he struck and his sword fell on both the man and the thighs of his wife; he killed the man and cut off the thighs of his wife.” ‘Umar took his sword and moved it back and forth and gave it to him and said: “If they do it again, repeat the same action; i.e. kill them”. This verdict of ‘Umar declared killing both this man and this woman lawful because there was doubtless proof that the murdered man was in fact having illegal sexual intercourse with the wife of the man who killed him. One can notice that ‘Umar did not differentiate between a married man and a single one and this proves that the declaration of impunity here is due to the transgression and the violation of honour.

The scholars are not agreed on the evidence that saves the person from being reattributed: Are two witnesses enough to prove his innocence or should there be at least four witnesses? The sound opinion is that whenever the evidence is doubtless (obvious), the retribution is cancelled and the person who kills somebody who violates his honor is not punished as done by ‘Umar . This is also the opinion of Imams Ahmad and Ishaq .

It is narrated in a sound Hadeeth that the Prophet said: “If someone is peeping (looking secretly) into your house without your permission, and you throw a stone at him and destroy his eyes, there will be no blame on you.” [Al-Bukhari]

It is also reported by Ahmad and An-Nasaai that the Prophet said: "Whoever peeps into a house without the permission of its dwellers, if they gouge out his eyes they are not subjected to pay any penalty or blood money."

All these Ahadeeth prove that any person who violates somebody's sanctity (honor) his, killing is lawful. From the above, we conclude that it is permissible for any man who finds another man having sex with his wife to kill him. But if this man does not have clear evidence for his claim (i.e., that he found the man he killed committing Zina with his wife), he himself is killed as a retribution. As for the woman, he should not kill her as she might have been forced by the man. But if he is certain of her complicity, killing her is like killing the man.

Killing a Muslim is a major sin and it is the most serious crime. Allah has threatened anybody who kills a Muslim of severe punishment. Allah Says (what means): {But whoever kills a believer intentionally - his recompense is Hell, wherein he will abide eternally, and Allah has become angry with him and has cursed him and has prepared for him a great punishment.} [Quran 4:93]

The Prophet said: “(I swear) By whom my soul is in His Hands, that if the whole world perishes, it is less important for Allah than the [unlawful] killing of a Muslim.” [At-Tirmithi]

The one who kills a Muslim deliberately is one of two cases: First, if he has killed the Muslim believing that such an act is lawful, he drives himself away from the fold of Islam and hence becomes a Kaafir. Second, if he has killed him believing that such an act is forbidden and abominable, but he killed him because of rage or other reasons.

In this case, he is not considered a Kafir according to the most preponderant opinion of the scholars. But he has committed a major and grave sin as stated earlier. But if he repents a repentance that meets all the conditions peculiar to it, Allah may forgive him for the evidence that repentance wipes out Shirk (polytheism), which is the gravest sin at all.

If the person who killed another person deliberately dies without making true repentance, he will be under the Will of Allah; if He wishes to forgive him, He forgives him; otherwise He throws him into Hellfire. The evidence for this is the sound Hadeeth that the Prophet said: “Give me the oath (Pledge of allegiance) for: (1) Not to join anything in worship along with Allah, (2) Not to steal, (3) Not to commit illegal sexual intercourse, (4) Not to kill your children, (5) Not to accuse an innocent person (to spread such an accusation among people), (6) Not to be disobedient (when ordered) to do good deeds.” The Prophet added: “Whoever amongst you fulfills his pledge, his reward will be with Allah, and whoever commits any of those sins and receives the legal punishment in this world for that sin, then that punishment will be an expiation for that sin, and whoever commits any of those sins and Allah does not expose him, then it is up to Allah if He wishes He will punish him, and if He wishes, He will forgive him.” So we gave the Pledge for that.”

Allah knows best.