There is a particular kind of horror that does not announce itself with violence. It arrives quietly, dressed in the ordinary language of duty, loyalty and order.

It is the horror of watching someone look at evidence of a massacre, filmed by the perpetrator’s own hand, and feel nothing.

No guilt. No revulsion. Not even discomfort. Just a blank, uncomprehending stare, as though the suffering belongs to a category of existence they have long since stopped recognising as real.

This is what confronted those present at the capture a few weeks ago of Amjad Youssef, the Syrian perpetrator who filmed himself participating in the Tadamon massacre in April 2013, when 41 civilians were thrown into a pit, fired upon at close range, and then had their bodies burned.

When authorities showed the footage to his family members - his own filmed testimony of slaughter - they did not recoil. They pleaded. They insisted he had simply been doing his job, carrying out his duties, following orders.