Inevitably, the Clinton campaign is calling for Samantha Power's head. That's to be expected, I suppose. But I think it's something of a pity. People in both campaigns say awful stuff about the other side all the time (though calling Hillary a "monster" was bracingly harsh, to be sure). So the content wasn't totally shocking.
Power's real mistake was declaring herself off the record a half-sentence too late. Because convention holds that a source must specify that before speaking, I think the reporter was within his rights to publish the quote. However on balance I wish he hadn't. Power is certainly a key Obama advisor, but she's an academic, not a politico, and also not a full-time spokeswoman for the campaign. (I would apply a completely different standard to someone like Howard Wolfson.) Moreover at the time of the interview she was in the UK promoting a book, not Obama per se. And what have we really learned here? We already knew these candidates are in a death struggle and their campaigns insult each other nonstop.
More to the point, when the media gives public figures zero margin for error, they offer us zero trust in return. There's a place for letting people vent their true feelings off the record in ways they can't publicly--and if they slip up, for not hanging them on a technicality. But each episode like this gives people yet another reason to say nothing interesting to journalists, which makes our understanding of the world less informed, which--although occasionally there will be a juicy story like this one--ultimately means readers are the poorer for it.