Is the act considered Zina and subject to Hudud punishment if a man has sexual intercourse with a woman, believing her to be his wife in the dark, and then it becomes clear that she is not? And is the husband of the woman who was sexually intercoursed held legally accountable if he kills the man who had sexual intercourse with his wife?

If a woman you thought was your wife had intercourse with you, and then it became clear that she was a foreign (unrelated) woman, there is no sin upon you in that; due to Allah Almighty's saying: "And there is no blame upon you for that in which you have erred, but [only for] what your hearts intended," and due to the Prophet, peace and blessings be upon him, saying: "Indeed, Allah has pardoned for my Ummah their mistakes, their forgetfulness, and what they are coerced into."