Is it permissible for a father to kill his married daughter by administering a lethal substance, such as poison, after she confessed to him and her family that she committed adultery with more than one person, in order to preserve the family's reputation and prevent the news from spreading among people?

A woman's father is not permitted to kill her, as the implementation of hudud (punishments) is the prerogative of the Muslim ruler or his deputy. Killing her would not conceal her matter; rather, it could lead to counterproductive results.

The woman is obligated to make sincere repentance, and she should be covered (her sin concealed). If she repents, her husband must keep her and treat her well. Some scholars are of the opinion that a married woman who commits zina (fornication/adultery) does not need to observe a waiting period (istibra') because it was narrated from Abu Bakr and a group of Companions (may Allah be pleased with them) that a woman who commits zina has no iddah (waiting period) at all and does not need to observe istibra', especially if she is married, due to the Prophet's (peace be upon him) saying: "The child is for the firash (bed/marriage)." In fact, the husband should have intercourse with her immediately so that no doubt remains in his heart. However, if the woman who commits zina is unmarried, she must observe istibra' by one menstrual cycle.

The family's pressuring her to confess is a grave wrong, for the Sharia (Islamic law) inclines towards concealment. It is recommended for one who confesses to zina to retract their confession, and the hadd punishment is dropped if they retract, even during its implementation, as the Prophet (peace be upon him) did with Ma'iz.