What Would Happen If The Israeli Army Killed Someone In The U.S.?

The Obama administration says it is ok for the U.S. to kill American citizens in another country, without a trial, indictment.
From the Washington Free Beacon:
White House press secretary said Tuesday the administration’s use of drones is “legal,” “ethical,” and “wise,” at a press briefing following remarks by President Obama.
“These strikes are legal, they are ethical and they are wise,” Carney said.
NBC News reported late Monday on an unclassified Department of Justice white paper on the use of drones against American citizens, like al Qaeda operatives Anwar al-Awlaki and Samir Khan.  http://freebeacon.com/carney-drone-strikes-legal-ethical-wise/
On the surface, it sounds like a sound policy, especially if the Americans canbe a danger to the United states.
However, how would the Obama administration handle a situation where a country sends their intelligence agency to the U.S. and kills one of their citizens on U.S. soil?  There would be investigations and outrage.
So, why would it be ok for the U.S. and not ok for another county?