I tried to sort out some thoughts in a private message, below, but couldn't come to any conclusions. Maybe someone else will have a good idea.
My way of dealing with this is the soothing rational argument that these events are less common than other killers like deadly bee stings, shark attacks, or lightning strikes. The problem is that we give these attacks so much attention. It's the media, etc.
But maybe the world is the rational one. These deaths disturb me far more than those from bee stings. Why? We are supposed to be adept at dehumanizing the out-group, and terrorists are the ultimate out-group. That term is the de facto label for anyone the authorities want to prosecute without retribution. (In the French press, the attackers are called kamikaze, which is weird.) Why don't we simply perceive suicide bombers and bombings as inhuman, unpredictable forces of nature, like volcanoes, that we just have to rue and tolerate?
Another thin comfort is that these attacks rely on a supply of people with a compromised instinct for self-preservation. I can't convince myself that this supply is not on the rise. I don't know why we don't see more of these attacks than we do, which would be good knowledge to have.
I tend to favor less strict border control, and like to think that migration and trade will mitigate the extreme differences that might fuel this kind of attack. But it's hard to formulate a convincing argument.
User b-612 tells us that "most of the terrorist had Belgian passports and been living in Belgium almost all their lives," and is rightly praised for caring about facts. Yet b-612 provides no citation, and my casual reading of coverage suggested that the authorities had only the usual fuzzy security photos and, I suppose, some shreds of meat to go on at the time b-612 made the comment.
Another in-group/out-group reflection is that a similar attack occurred recently in Ankara. I see no mention on Hubski (though search is not reliable). A British expat got some shares on the other social network asking "You were Charlie, will you be Ankara?" The Turks, long standing at the door of the European Union, chafe at the perception that they are not good enough for the European "Christian Club." The question of whether an Us-Them mentality contributes to an atmosphere in which mistrust and hatred festers is a touchy one. Rebuffed by the West, the present regime has turned east, cozying up to Russia and Iran, to the consternation of many Turks. The arrest in Miami last week of a previously untouchable businessman/insider made a U.S. Attorney a Twitter sensation in Turkey. Reza Zarrab is accused of evading sanctions by paying Turkey's gas bills from Iran in gold, with boxes of dollars stashed on the side.
For now, b-612 looks more and more attractive, despite two active volcanoes and one extinct.
Mais, comme il disait, "On ne sait jamais!"