Old images of violence returned to haunt us this week. Two, in particular, have become symbols of the last decades of the century.
The first is the image of a hijacked plane on an airport runway as the innocent find themselves used as pawns in someone else's game. The second is that old black and white image of Moors murderer Myra Hindley, hospitalised with a suspected stroke.
The hijacking and the murder of - at the time of writing - one passenger will have left our minds by this time next year. But we'll still remember Myra Hindley.
Yet if you were to produce a league table of horrors, those who kill for a cause greatly outdo the Hindleys. Think of Bosnia, Kosovo, Northern Ireland.
One way to look at violence is to see it as either moralistic or predatory. Moralistic does not mean moral. It means violence arising from a conflict in which you see the other person as being in the wrong.
If you kill someone, it is likely to be for moralistic reasons: they have built a wall 6in into your property; they have had an affair with your spouse; they have overtaken you and cut in front of you and caused you to crash. You are right and they are wrong and that is your justification.
Predatory violence would include killing someone because you get a kick out of doing so. The crimes of Hindley and her partner, Ian Brady - who between them murdered at least five children and teenagers - were predatory. So was the murder of Jamie Bulger by two older boys.
Predatory violence also includes rape, killing someone for their money, and other horrors with which we are too familiar. The crimes of Father Brendan Smyth provide another example.
It is predatory violence that horrifies us most, that grabs media headlines, and that is the staple of crime movies and novels.
But it is moralistic violence that accounts for the vast majority of murders.
This holds across time and cultures. For instance, one researcher, Barbara Hanawalt, studied coroners' records for Northamptonshire and London between 1300 and 1415. She found that of 459 homicides, only 20 per cent were predatory i.e. committed to further a robbery. The other 80 per cent arose from interpersonal conflict and were moralistic.
In more recent times, the records of 22,000 homicides reported to police in Chicago between 1965 and 1993 show that only 19 per cent were committed to acquire money or property. At least 64 per cent can be described as moralistic. That is probably the opposite of the impression we get of the causes of murder in the United States.
Other studies in countries as diverse as Uganda and India show that the overwhelming majority of murders are moralistic. Indeed in the poorer, more tribal societies, almost all are moralistic.
I am indebted for much of this information to Mark Cooney, an Irishman. As assistant professor of sociology and adjunct assistant professor of law at the University of Georgia, he is the author of a fascinating study on the topic, Warriors and Peacemakers, published by New York University Press.
His book left me with the impression that we are most likely to do wrong when we are most convinced we are right.
I am sure many of those who inflicted cruelties on children in their care in institutions in the past believed they were doing the right thing. In an era when children were seen as having a tendency towards wrong, these particular children were set up for extreme treatment.
The most unfortunate also fell victim to predators who took the opportunity to satisfy their sexual or sadistic inclinations. Paradoxically, the "moralistic" atmosphere gave them their opportunity to abuse children.
The attitude that somebody has wronged you and that you are entitled to retaliate with extreme violence can arise in a matter of seconds. Look at the number of murders or violent non-fatal attacks which take place after a row in a disco or a pub. A conflict arises, each side sees the other as in the wrong, and a savage attack follows.
But moralistic crimes can be a long time in preparation, too, it seems to me. The Holocaust was preceded by centuries of blame heaped on the Jews for everything from plague to the disappearance of children to economic misfortune. It all led to the concentration camps and the gas chambers. And before that it led to many mini-holocausts in many places over a long, long time.
How many words spoken over how many decades created the moralistic atmosphere which still saturates Northern Ireland? Was there ever a place in which people were more insistent that they are right and the other side is wrong? We have seen the results of that. The fear is that we will see more, not in the form of paramilitary violence, but in individual attacks fuelled by the same moralistic fervour.
Words matter. Words of hate are like bullets that kill in the future in the ways speakers had never imagined.
Whoever sprayed "Kill a nigger for Xmas" on a wall in Dolphin's Barn a few weeks ago may get his or her wish. It may not come next Christmas or the Christmas after that. But the words, and the ideas behind them, may do their own little bit to create a moralistic attitude which will one day justify murder.
This is not to say that people should not express their views on the issue of asylum and immigration. But the way in which they express those views matters. So does their degree of tolerance of other people's opinions. Both will matter a great deal to the sort of atmosphere which their words will, whether they like it or not, help to create.
The Myra Hindleys, the Fred and Rosemary Wests, and the other predators will always be the stuff of nightmares. They stand so much to one side of normality that we cannot comprehend them.
But it is those who are like us, who have a sense of right and wrong, who do the most damage in the end. That is the terrible, terrifying human paradox with which we have to come to terms.
pomorain@irish-times.ie