Why do Tanner and Darrow get into a fight?

A group of refugees walks alongside the road. Tanner and others are also present. Tanner goes to take a photo of an emaciated young man pulling a cart filled with provisions, grandparents and children. Darrow loses his temper and a fight is barely avoided. Tanner says that Darrow is a lunatic. Darrow later tells Helen that he did not care about the people for a long time. Tanner crossed the line in Darrow's eyes by exploiting the people.