Who would take over the presidency if everyone dies?

I understand your question. During the State of the Union a lone HR or Senator takes of and crashes into the Chamber killing all. I think that the Chief Justice of the Supreme Court would take over unless they are present also. If that is so then the Highest Ranking Military Officer I think would take charge. Or there is the possibility that the Mayor of Washington D.C. would be the person to take over. After that I don't know. Hope this is what you were looking for.