Pray for the dead and for the recovery of the injured. And don't give the hard men of the daesh, who sent the killers, what they want.
The daesh exists now because the United States responded to the terror attacks of 9/11 with an ill-advised invasion of Iraq. That invasion and the subsequent occupation destabilized a long standing sectarian quarrel, and as a result a particularly murderous group (the daesh, who sometimes call themselves the "Islamic State") gained adherents. On June 29 2014, they proclaimed a Caliphate, serving notice they aspired to worldwide political and spiritual authority.
After some initial military successes from 2013 to 2015, they suffered a number of defeats. These defeats do not merely cost them political power and the lives of their fighters; military defeat puts the basis for their claim of political power, their religious claims, into doubt. This failure has implications not only for the relatively small and local movement behind the daesh; it puts Islamist ideas into doubt. The surviving leaders of the daesh have now attempted, in a desperate effort to salvage a victory, a campaign of terror. Like all campaigns of terror, the actual deaths it causes amount to a tiny fraction of the regular death toll from road crashes.
Driving a van into innocent people in Barcelona will not stop the Kurds, Shiites, and the official governments of Iraq and Syria from driving back the daesh and eliminating their political control over territory. The surviving leaders of the daesh know that. They hope their violence will trigger ethnic and religious violence in response, and lead to a general religious war. That offers their last hope to avoid utter defeat.
The daesh clearly want to provoke a general war. They will almost certainly fail. That does not stop a few ill-informed or mischievous people putting calls to restrict the rights of all Muslims, ranging all the way to proposals to ethnically cleans Muslims wholesale from Europe. in various web fora. I can think of few things more calculated to inflame Muslim public opinion and enlist young Muslims in the kind of war the daesh desperately needs than images of Europeans perpetrating ethnic cleansing. A call not for censorship but responsibility makes sense here; if every time the daesh perpetrates a rampage of the sort we saw in Barcelona someone proposes ethnic cleansing of Muslims in Europe, the leaders of the daesh may hope one more outrage will provoke the general war they badly want.