In the study, he didn't ask his subjects to kill. And they didn't actually murder anyone.
> Rwanda ... Cambodia ... Nazis ... Interahamwe
People killing on behalf of the State is a whole other matter than what the article is talking about vis-à-vis pedestrian fatalities. The State has been the largest single driver of misery, destruction, and death in human history, and I don't disagree that mass murder has happened and is evil. But drivers ensuring the death of a struck passenger stands in stark contrast to the kill-or-be-killed nature of State violence. On this point, I would agree with your assertion about "compelling justification," which in the case of the State is the threat of violence against the killer for not doing the bidding of the State.
But even in the face of multiple genocides and huge wars, including nuclear bomb blasts, the fact remains that the vast majority of humans have never and will never kill another human.
> A great number of Americans ...
See a sibling response for murder rates and compare to my assertion that a vast majority of people don't do it.
And self-defense is not murder by the legal definition, which include premeditation.
[0]: http://www.historynet.com/men-against-fire-how-many-soldiers...
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.