In saga-era Iceland, killers had three times as many biological relatives and in-laws as their victims:
In the three sagas, a total of 66 individuals caused 153 deaths; two or more attackers sometimes participated in the same killing. No killers were biologically related to their victims (such as cousins or closer), but one victim was a sister-in-law of her killer.
About two-thirds or more of killers had more biological kin on both sides of their families, and more in-laws, than their victims did.
Six men accounted for about 45 percent of all murders, each killing between five and 19 people. Another 23 individuals killed two to four people. The rest killed once. Frequent killers had many more social relationships, through biological descent and marriage, than their victims did, suggesting that they targeted members of families in vulnerable situations, the researchers say.
The Icelandic legal system depended on private initiative. More precisely: when someone was found guilty, the court gave the victim’s family the right to retaliate.
David Friedman has written on the private creation and enforcement of law of saga-era Iceland.