What makes an armed man kill an unarmed civilian? What makes him join the killing of many hundreds — or hundreds of thousands — of civilians?

University of Amsterdam Professor of Social Science Emeritus Abram de Swaan takes a hard look at the subject in his new book, “The Killing Compartments: The Mentality of Mass Murder.” History is appallingly dense with examples, from medieval warfare to Nazi Germany to Stalin’s purges to Rwanda in the 1990s.

De Swaan talked with the Gazette ahead of a Tuesday lecture at the Minda de Gunzburg Center for European Studies.

GAZETTE: You point out that genocide did not arise in the 20th century. Has it been with us always?

DE SWAAN: I’m afraid so. I avoid the term “genocide” because it has a very precise but rather problematic legal definition. I use “mass violence” for very large-scale, asymmetric encounters between organized and armed men — it’s usually men — and organized and unarmed other people.

In many times and places, a victorious army would routinely kill off whoever it could get ahold of, rape the women, enslave people, burn. Until, say, the 15th or 16th century, the chroniclers exaggerate the amount of bloodshed with glee and pride in what their army has accomplished and how much carnage was [done] in honor of the king or the great leader.

What is modern about genocide or about mass violence is the embarrassment about it. ...