He was talking about undertaking large activities (war for example) that would kill hundreds of thousands of people could be tolerated politically by a populace

He was talking about undertaking large activities (war for example) that would kill hundreds of thousands of people could be tolerated politically by a populace in a way that a single murder might cause an outrage. The large numbers make the horror abstract. It's easier for people to relate to a story about a specific individual. The quote (and variations) are quite famous, it might be apocryphal, but I think it is true. Hearing about hundreds of thousands of people massacred in a foreign land doesn't hit home when you are reading about far away. That's why reporters (New York Times style)try to weave in illustrations and stories about individuals even when discussing a larger trend.