"Unwritten Law" did exist! Astonishing

I always assumed the "unwritten law", that is, a husband could kill his wife's lover if caught in adultery, was just an urban myth. 

The suggestion that provocation, a legitimate defence used today, is the modern form of "unwritten law" fails as provocation does not justify homicide - it merely goes to degree of guilt - and is not directed at adultery at all. 

Both Blackstone and Hale expressly held there was no justifiable homicide at Common Law if a man killed his wife's lover. 

But some United States jurisdictions did write the "unwritten law" and those statutes remained into the 1970s!

Here's a summary from Jeremy D. Weinstein, Adultery, Law, and the State: A History (1986):

Some American states, either by statute or judicial decision, made it legal for a husband to kill an interloper caught in the act of adultery with his wife. [Footnote: The civil law provides an interesting comparison. Its primary difference from the common law is that since ancient times the cuckold was allowed to kill the wife as well as the marital interloper.] ...

Until 1974, when it was repealed, [a] Texas statute provided:

Homicide is justifiable when committed by the husband upon one taken in the act of adultery with the wife, provided the killing take place before the parties to the act have separated. Such circumstance cannot justify a homicide where it appears that there has been, on the part of the husband, any connivance in or assent to the adulterous connection.

... Although an early case established that the statute permitted the husband to kill his wife as well as her paramour, Texas courts criticized this interpretation and reversed it the following decade [in 1925]. Furthermore, Texas judges refused to extend the statute to permit a wife to kill her husband's paramour. Under the Texas statute, the injury to the paramour was only justifiable when inflicted with the intent to kill [as opposed to, in one case, castrate]....

Until 1977, Georgia [common law] also permitted a husband or father to kill the paramour of his spouse or child under limited circumstances.... The Georgia courts interpreted the justifiable homicide law as a class of self-defense, while the Texas statute was in effect a law allowing revenge. The most significant distinction between the Georgia and Texas rules was that the killing under the Georgia rule was defensive in nature and had to be necessary to prevent and defend against the adultery. A killing after the adultery was vengeance, and therefore was murder or manslaughter depending on whether it was committed in the sudden heat of passion. The defensive nature of the justification allowed killing to stop an adulterous relationship of which the husband or father was aware if it seemed to the husband or father to be the only way to do so. In contrast, under the Texas statute a husband was only permitted to act if he was surprised with a present adultery.... [The law] extended to the protection of daughters and fiancees, although evidence of the woman's chastity was admissible on the question of whether it was necessary to kill to protect it, which was a question for the jury.

In contrast to the situation in Texas, in Georgia the wife could kill her husband's paramour, but just as in Texas, the spouse was not permitted to kill the other spouse. Another difference between the Georgia and Texas justifiable homicide rules was that, in Georgia, the paramour [generally was not allowed to use lethal force to defend himself against the spouse's lethal attack]....

[Until 1973], two other American jurisdictions, New Mexico and Utah, had statutes justifying the killing by a husband of his wife's paramour if he found them together in adultery. [Unlike the rationale given by Georgia courts, the New Mexico courts framed the matter as one of excuse rather than justification]: "[t]he purpose of the law is not vindictive. It is humane. It recognizes the ungovernable passion which possesses a man when immediately confronted with his wife's dishonor. It merely says the man who takes life under those circumstances is not to be punished; not because he has performed a meritorious deed; but because he has acted naturally and humanly."