Our morals aren't nearly as absolute as we like to think. 20% of the entire population of Rwanda were hacked to death with farm implements by ordinary people. 25% of the Cambodian population were butchered, starved or worked to death by a poorly-organised militia.
Any of us have the capacity to kill if we have a sufficiently compelling justification. Ask any mother if she would kill to save her children. The difference between murder and self defence is mere context - the Nazis believed that they were defending themselves from a Jewish conspiracy, the Interahamwe believed they were liberating themselves from Tutsi oppression.
Consider the fact that the rate of domestic homicide in the US is drastically higher in households that own a gun. A great number of Americans have killed their partner simply because the means to do so easily was readily available. Evil or insanity is not a prerequisite for murder, only means, motive and opportunity.
