What is the difference between the hadith "If you kill him, we will kill you" and "Whoever dies defending his family is a martyr," and what is the ruling of Islamic law on someone who finds a stranger in his house and kills him?

The statement by the questioner, "If he killed him, we would kill him," refers to what Muslim and others narrated regarding a man from the Ansar who asked the Prophet, peace and blessings be upon him, about a man who found another man with his wife. The Prophet's response was, "He should speak, and he will be lashed. And if he kills him, he will be killed." In another narration, "O Allah, open [the way]," whereupon the verses of li'an (mutual imprecation) were revealed.

Most scholars are of the opinion that whoever kills a man he finds with his wife shall be subject to qisas (retaliation in kind), even if he is not held accountable between himself and Allah if the slain man was married (muhsan) and had intercourse with the woman in a way that necessitates ghusl (ritual bath). Ahmad and Ishaq hold the view that his blood is not avenged if he presents clear proof, while Al-Shafi'i believes that it is permissible for him to kill him between himself and Allah if the woman was a thayyib (previously married), but qisas is not waived in the apparent legal judgment.

This ruling does not contradict the hadith, "Whoever is killed defending his property is a martyr, and whoever is killed defending his religion is a martyr, and whoever is killed defending his life is a martyr, and whoever is killed defending his family is a martyr"; because martyrdom pertains to the afterlife, while qisas is among the worldly legal rulings.