The hate and animosity of a blood feud extends beyond simply killing the kin of someone who killed one of yours. It also involves punishing, in any way you can, anyone associated with the killers. In Iraq the cycle of revenge was started long before the U.S. and Britain invaded in 2003. For several decades the Sunni Arab minority (20 percent of the population) had become increasingly brutal towards the majority Shia Arabs (60 percent), the Kurds (15 percent), and several smaller minorities. The Shia and Kurds suffered hundreds of thousands of deaths.
Once the Sunni government was overthrown and the Sunni controlled police, secret police, and military disbanded, the Shia got organized and began going after individuals they knew had killed specific Shia men, women, and children (usually in gruesome fashion). The newly elected government and the foreign troops tried to shut down these death squads, but that did not happen in a big way until the Sunni tribes agreed, in 2007, to turn against the Sunni Islamic terrorists. Even with the Shia militias and their death squads put out of business by 2008, there were still plenty of Shia men, or groups from the same family or tribe, out looking for the Sunni bastard that killed, maimed, or raped someone close. This vengeance was the main reason so many Sunni Arabs fled Iraq after 2003. The vast majority did not fear prosecution for war crimes but rather the inexorable progress of a blood feud.
In addition to seeking out and murdering those who killed someone close, it’s also common to inflict all manner of pain on the family of the killers. Thus the widows of known Sunni terrorists are often prevented from obtaining any government benefits. This is usually done by exploiting the fact that terrorists who married while they were terrorists were often not able to register the marriage (and the subsequent children). The widow and children are in constant fear of being discovered by kin of her late husband’s victims. Since her male children will feel compelled to carry on a blood feud to avenge those who killed his father, the male children are sometimes killed, especially if the kid does not come from a large extended family that might contain cousins or uncles who would start a blood feud to avenge the murder of the child.
Islamic terrorists find it easier to recruit young men who are eager to kill someone as part of a blood feud. As men get older, marry, and have children working the blood feud declines in importance. In old age it becomes a distant memory, unless someone from the other side of the feud gets to you. But when a guy is young, killing someone in a feud is a quick way to get some recognition and prestige. It’s also a quick way to get yourself killed, but young men tend to downplay that part.