Excerpted from here
Elie Wiesel (1985, Vol. 1, p.33) gave an answer to this haunting question with respect to Jewish Holocaust.
‘All the killers were Christian. The Nazi system was the consequence of a movement of ideas and followed a strict logic; it did not arise in a void but had its roots deep in a tradition that prophesied it, prepared for it, and brought it to maturity. That tradition was inseparable from the past of Christian, civilized Europe.’