What is the ruling on a husband who kills his wife and her partner while they are caught in the act of adultery, and is he to be killed in retribution (qisas) or as a legal punishment (hadd), or is he to be left alone, and what is the evidence for that, and do the scholarly opinions differ on this matter?

If one finds a man with his wife, he must wait until he brings four witnesses to the act of adultery. If he kills him without witnesses, then he is subject to retaliation (Qisas). His statement is not accepted without proof, because the default assumption is the absence of what he claims. However, if the guardian [of the deceased] admits to it, then there is no Qisas or blood money (Diyya) due from him.