Tashfeen Malik is pictured in this undated handout photo provided by the FBI, December 4, 2015. U.S.-born husband, Syed Rizwan Farook, 28, and his spouse, Tashfeen Malik, 29, a native of Pakistan who lived in Saudi Arabia for more than 20 years, died in a shootout with police hours after Wednesday's attack on a holiday party at the Inland Regional Center social services agency in San Bernardino, about 60 miles (100 km) east of Los Angeles. REUTERS/FBI/Handout via Reuters ATTENTION EDITORS - FOR EDITORIAL USE ONLY. NOT FOR SALE FOR MARKETING OR ADVERTISING CAMPAIGNS. THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. IT IS DISTRIBUTED, EXACTLY AS RECEIVED BY REUTERS, AS A SERVICE TO CLIENTS

Thou shalt not kill!

Tashfeen Malik is pictured in this undated handout photo provided by the FBI, December 4, 2015. U.S.-born husband, Syed Rizwan Farook, 28, and his spouse, Tashfeen Malik, 29, a native of Pakistan who lived in Saudi Arabia for more than 20 years, died in a shootout with police hours after Wednesday's attack on a holiday party at the Inland Regional Center social services agency in San Bernardino, about 60 miles (100 km) east of Los Angeles. REUTERS/FBI/Handout via Reuters ATTENTION EDITORS - FOR EDITORIAL USE ONLY. NOT FOR SALE FOR MARKETING OR ADVERTISING CAMPAIGNS. THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. IT IS DISTRIBUTED, EXACTLY AS RECEIVED BY REUTERS, AS A SERVICE TO CLIENTS

Tashfeen Malik is pictured in this undated handout photo provided by the FBI, December 4, 2015. U.S.-born husband, Syed Rizwan Farook, 28, and his spouse, Tashfeen Malik, 29, a native of Pakistan who lived in Saudi Arabia for more than 20 years, died in a shootout with police hours after Wednesday's attack on a holiday party at the Inland Regional Center social services agency in San Bernardino, about 60 miles (100 km) east of Los Angeles. REUTERS/FBI/Handout via Reuters ATTENTION EDITORS - FOR EDITORIAL USE ONLY. NOT FOR SALE FOR MARKETING OR ADVERTISING CAMPAIGNS. THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. IT IS DISTRIBUTED, EXACTLY AS RECEIVED BY REUTERS, AS A SERVICE TO CLIENTS

The most basic human moral responsibility is to train people not to murder each other. It appears that we are increasingly failing at this task in the United States, as our almost weekly mass murders attest.

As an ethicist, one of the things I study is how communities understand the nature and basis of moral obligation -- the understanding of what is right and wrong, good and bad, obligatory and forbidden, and why.

One of the most widely shared moral norms in human history is a ban on murder; that is, unjust killing. Across millennia and cultures one generation has trained the next to restrain itself from the shedding of innocent blood. Murder usually has been treated as the single gravest offense and has been met by the most serious punishments. These efforts have been successful enough to enable human communities to survive.

In historic Christian civilizations, the ban on murder was grounded in biblical commands and therefore in the will of God. "Thou shalt not kill" is written into the Ten Commandments and repeated all over the Hebrew Bible, which became part of Christian Scripture as well.

The addressee of this command was understood to be not just Israel or the Church but every human being. The speaker was understood to be God. The punishment for murder was death. The eternal punishment for murder was worse -- eternal death; that is, going to hell where one suffered divine wrath forever.

Of course Christian faith offered more positive reasons as to why one must not murder. All clustered around the idea of the divinely given worth of human life.

The Bible and later Christian tradition are filled with increasingly expansive claims concerning the sacred worth of each person. These claims intend to place a zone of protection around humans simply because they are the unique, immeasurably valuable creatures they are. Because God looks at people this way, we must act accordingly. At a minimum, this means restraining our hand from shedding innocent blood. (Legitimate defensive war has been an exception; more below.)

So there were at least three fundamental ways at least one civilization tried to train people not to kill each other: a) by threatening eternal divine punishment; b) by enacting earthly execution; c) by elevating the value of human life as such.

Of course, one did not have to be a moral giant to be able to adduce other reasons not to kill. A bit of empathy helped -- if I kill this man, he will suffer and his family will grieve deeply. A minimal sense of the common good helped -- if everyone murdered when they were angry society would collapse. Maybe just a bit of Golden Rule -- I will not kill this man because I would not want to be killed. And some chivalry when it came to women and children once resonated -- a real man doesn't kill women and children, ever.

I am not attempting to whitewash the many, many occasions in which Christians failed to live up to their own code. But I am saying there was a code, and it was widely understood. And I am wondering whether it has collapsed, or is being threatened by a very different code.

Robert Dear, suspect in Planned Parenthood shooting in Colorado Springs

Robert Dear, suspect in Planned Parenthood shooting in Colorado Springs

Collapsed: Think of the garden-variety aggrieved white American guy who shoots up a movie theater because he is depressed or angry or mentally ill. Such a person, at least in that moment, appears to fear neither God nor man, shows utter disrespect for human life as such, and cares nothing for the suffering he causes and the injustice he does. We need to figure out how people get to such a place, and prevent it.

Threatened: The religiously inspired terrorist-murderer represents a different, more ancient kind of moral and indeed theological problem. Today they are primarily emerging from radical Islamist groups, but don't overlook radicalized Christians, Jews, and others.

These persons believe they are warriors in a holy cause, thus their violence is not murder but legitimate war against God's enemies. They kill not in the teeth of anticipated divine punishment but with a twisted sense of divine blessing. They do not fear execution or death because they believe such death to be martyrdom and they anticipate eternal reward. They do not respect human life as such but only those human lives that conform to their version of their religion. And they suppress empathy, justice, common good, chivalry, Golden Rule, or other moral restraints in pursuit of their holy (e.g., desperately unholy) cause.

Any society that discovers it is being afflicted by attacks from either of these types of murderers must protect itself by all prudent and lawful means. That is primarily a government responsibility. Certainly it includes keeping military arsenals out of the hands of potential murderers.

But the broader problem is moral and religious. All of us bear responsibility for (re)creating a moral world in which murder, not to mention mass murder, becomes morally unthinkable, however it might be motivated. Religious communities that find they are consistently producing murderers must recognize the internal crisis they face, and act to redress it immediately.