An interesting backgrounder on honor killing is in Forbes magazine, of all places.

A much older code than Islam is at work here, one that Islam doesn’t entirely repudiate as it tries to accommodate and sanctify tribal tradition. The more innocent the victim, the more she must be sacrificed because the curse is all the more palpably divine for being arbitrary. Muslim conservatives often try to retro-justify the situation by assertions that the girl no doubt behaved lewdly or failed to follow parental rules. Even if true, what law was the rapist following? The answer you will get, literally, is this: He was following the law of nature. He was possessed with desire, spontaneously and irretrievably, couldn’t curb himself, shouldn’t be blamed. In effect, he was possessed by the gods, or the Djinns, and became their instrument. He deserves merciful understanding.