A family of Canadian volunteers dedicated to alleviating poverty in Africa. A group of intrepid German retirees on a tour of Turkey and the Middle East. An Iraqi who had gone to Baghdad seeking refuge from the jihadist violence of his hometown. A Canadian audiologist who had fallen in love with Indonesia.

They were among the scores of people slaughtered by Islamic extremists in four countries last week in spasms of bloodshed that left loved ones stunned at the randomness of the killings.

“It will never be understood: ‘Why you?’ No one can give an answer,” observed André Franke, a relative of one victim, Karin Franke-Dütz, 70, a retired teacher who was among the Germans killed by a suicide bomber in Istanbul. Mr. Franke summed up a universal anguish in a Facebook post, saying, “Incomprehensible that we lost such a cordial, wonderful person in such a terrible way.”