The sight of thousands of young people in London celebrating the massacres that Hamas committed in Israel unsurprisingly made many people’s blood run cold. The demonstrators rejoiced not despite the brutality of what was done but because of it.

What must the celebrants have believed to celebrate in this fashion? It was impossible that they were uninformed as to some of the details of what Hamas had done; nor did they deny the reality of these atrocities. If Hamas had merely sent rockets into Israel that destroyed some, or even many, buildings, there would not have been the same rejoicing. It was the brutality and sadism—the beheaded or burnt babies—that made the difference and was the cause of so much pleasure and joy.

A friend who has spent many years talking to Arabic-speaking supporters of the Palestinian cause in Britain (he is a professional translator) told me that he was not in the least surprised by the celebrations. The supporters had long had what might be called a genocidal imagination. Annihilation of a population, not victory over a state, was their dreamed-of solution.

There has long been a tendency in some intellectual circles to believe that the justice of a cause must be proportional to the lengths that people are willing to go to promote it. Only very desperate people, the argument goes, would do such things; therefore, since they do such things, they must be desperate.