Does America Have a Responsibility to Stop Mass Killings?

Yes and no, but ultimately, as Friedersdorf points out, the decision is up to Congress:

I think it's Congress that should decide whether America has a "responsibility to protect" in a given instance. That is partly because I am a stickler for acting in accordance with the Constitution, a rare quality these days. But it's also because, like the Founders, I believe that presidents would be too inclined to start wars if given the prerogative; that the most legitimate way to decide if it's justified to use American resources when our national security interests aren't at stake is to put the question before the representatives of the people; and because the legislative branch is less likely than the executive to misrepresent facts on the ground in a target country.

(This is in response to a panel discussion in Aspen that included Jeffrey Goldberg and Anne-Marie Slaughter.)