How are we to understand "honour" killings?

I was well aware that it happened, but I had no idea of just how rampant and widespread so-called "honour" killings were in Pakistan and in other benighted countries where these tragedies occur.
My attention was drawn by reports of a recent example in which Maria Sadaqat, a young schoolteacher in a small hill-town near the Pakistani capital of Islamabad, was beaten and then burnt to death by a group of townsmen for turning down a marriage proposal from the son of the owner of a school where she had taught. The intention apparently was for Ms. Sadaqat to take over the running of the school but, bizarrely, the owner's son at the time of the marriage offer was already married with a daughter, which seems to me like a perfectly good reason for Maria and her father to refuse the marriage offer. However, the school owner and other religious zealots of the town did not see it that way, and decided to take things into their own hands (the school owner himself was one of the aggressors in the murder, and apparently proud of the fact). The so-called elders of the town are now pressurizing the girl's family to settle out of court, and to respect the "honour" of the situation.
This of course is just one of many such occurrences in Pakistan, India and many other countries where fundamentalist religious convictions hold sway. What I hadn't appreciated, though, is the extent of it: Pakistan's independent Human Rights Commission estimates that 1,100 women were killed in the country last year in so-called honour-killings. The United Nations estimates that 5,000 women are victims of honor killings each year worldwide, mainly in countries such as Egypt, Jordan, Lebanon, Morocco, Pakistan, Syria, Turkey, Yemen, as well as other Mediterranean and Persian Gulf countries, and even within migrant communities in western countries like France, Germany and the UK. Women's advocacy groups, however, suspect that the real number may exceed 20,000.
The other problem of course is that most occurrences go unreported and uncounted (they are more likely to be reported as suicides or accidents), and the statistics do not include non-lethal honour attacks such as acid-throwings, abductions, mutilations, beatings, etc (both reported and unreported). Most honour killings are carried out by family members and relatives, but a small number are also carried out by people outside the family who perceive some loss of honour for the family or for the community as a whole. The main reasons behind honour killings are domestic disputes, alleged illicit relations, and exercising the right of choice in marriage.
It seems inconceivable to us in the West that these kinds of benighted attitudes still prevail in other parts of the world (not that we have our own shit completely together, but at least we have managed to leave some of our more egregious medieval customs behind). I have never actually heard anyone trying to justify such practices as honour killings, but I imagine it would have to be couched in religious or quasi-religious mumbo-jumbo that would mean very little to me anyway, and that would in turn beg its own (non-existent) justification.