1) On the merits, this is outrageous and a tragedy. One of the first principles of diplomacy is that nations have a duty to protect the representatives of foreign states sent there to do international business. Today, American security forces have a duty to protect Libya's diplomats at their embassy in Washington DC and in consulates elsewhere. Yesterday, Libyan security forces had a duty, which they did not fulfill, to protect Ambassador Christopher Stevens and other U.S. representatives in Benghazi.
2) On the short-term politics, one reason why people who have seen previous campaigns always insert, "Anything could happen, but..." when giving forecasts about presidential races is that, indeed, anything could happen. Political races and policy arguments grind their way along, economic trends push slowly in one direction or another, and then from time to time wholly unforeseen events occur. The political ramifications of this event in the United States are nowhere near its most important consequence. But this counts as one of the wholly unforeseen events affecting the political cycle.
3) On the longer-term temperamental politics, this is a very vivid example of what people mean when they talk about "the 3 a.m. phone call." In these next few hours let us look very carefully at the first-reaction quick responses, and then the considered second-take positions, by the two candidates.* One or the other of them will be in charge of U.S. response to similar inevitable-surprise episodes in the next four years.