One afternoon last December, an assassin on board a K.L.M. flight from Mexico City arrived at Amsterdam’s Schiphol Airport. This was not a business trip: the killer, who was thirty-three, liked to travel, and often documented his journeys around Europe on Instagram. He wore designer clothes and a heavy silver ring in the shape of a grimacing skull. His passport was an expensive fake, and he had used it successfully many times. But, moments after he presented his documents to Dutch customs, he was arrested.