The mass killing in Norway one week ago today has prompted the same hard question in varied forms around the world: how can any country prevent extreme political or social views from turning into violence?
No one will forget the pictures of Anders Breivik standing on the shore of Utoeya Island shooting at teenagers in the water. We need hardly say that the killing of young campers is repugnant to sane people everywhere, violating as it did the basic standards of humanity.
But few can argue that any act of violence is always wrong. And the ancient, universal concept of the "just war" has led to a remarkable range of tactics in the era of asymmetrical conflict.
But was Breivik truly politically motivated? To be sure, he left plenty of proof that he has become thoroughly polluted by the racial, religious and cultural hatred that flourishes on the internet. He knew all about the Unabomber and the 1995 Oklahoma City bombing. And Breivik was full of hateful rhetoric about Muslims (which makes it doubly sad that so many assumed, at first word of these atrocities, that Muslims were to blame).
All of that helps to explain why many observers are using the word "evil" about this killing spree. That word is easy to misuse, however - remember George W Bush and his "axis of evil"? - and in this case the word "insane" may prove more suitable. Was it sane, by any definition, to kill non-Muslims to express an already irrational hatred of Muslims?
Some commentators have chosen to use this tragedy to condemn the whole gamut of European political parties critical of immigration. Most European countries do have such parties, which play on social fears about immigration trends and demographic change. But one violent crank does not stand for every immigration sceptic.
Arab and Muslim peoples know full well how unjust it is to let a few murderers define the whole. Al Qaeda has laid claim to Islam while committing vile acts - killing civilians, forcing mentally impaired children to become suicide bombers, the list goes on - that are also evil, or perhaps insane.
Breivik admired Islamist terrorists, for they are two sides of the same coin. It is not their ideologies that define them, but their murderous tactics that the rest of us inherently know are wrong.
In a world where so many commodities can become a weapon, there are no obvious solutions to this threat. But in every society most people know that whatever you think, there are some things you must not do. As long as we have that consensus, the Breiviks and bin Ladens are a feeble force indeed.
