Assume Leonardo Da Vinci invented a scientifically wondrous piece of equipment: a vaporizer gun. The gun is able to instantly vaporize any human being or animal, leaving absolutely no trace behind - all clothes, possessions, fake teeth, etc, disappear without a trace. Since the vaporization only takes 1/1000th of a second, it is also a completely painless death.
Would this mean that murder and genocide become more acceptable in the human culture?
The massacre of the Armenians would leave no evidence behind: no photos, no graves, no horrible stories. The Holocaust would lack the concentration camps as people would be vaporized in their homes, without even realizing what is about to happen. Pol Pot would use the vaporizer guns to carry out his policy rather than starving people to death and forcing them into exile.
My thinking is that without evidence it would be impossible for the modern world to realize the horrors of genocide and it would become a rather distant triviality.