people who consider themselves civilised

The impact of the Rwandan tragedy or of Hiroshima on public consciousness is no doubt due to the fact that each of these slaughters took place within a short period of time. But if a system of domination regularly produces so much death and suffering, is the horror any less just because it stretches over a longer period of time? Shouldn't it be surprising in the post-1945 world, where racism was officially discredited and abolished, people who consider themselves civilised have killed so many people they consider not civilised enough?