What happens of the President, Vice President, Speaker of House, and the rest dies?

it would take a lot of time to kill all those people and im sure the government would figure out what was going on before everyone died, but if eveyone did die there is a simple answer to your question, we would all be screwed, thats it, no one would be able to agree on what to do next, the people with the most physical force would take control, and we would basically have a dictatorship, if everyone died that was suppose to lead our nation we wouldnt have to worry, because it wouldn't help us a bit to worry, it would be like what happened afte katrina where people where robbing stores to get what the wanted or needed except on a national scale because as Americans, most of us are to afraid of our own shadows to confront a major problem head on