VERY DARK WHAT IF. What If left and right extremists nutjobs combine and kill all prez candidates and the prez? What happens next?

The Dems find themselves another batch or candidates that look and sound identical to the troupe that's in the race today, President Pence is the GOP standard bearer and the media literally self destructs over the hysteria produced by such an event.

Somebody would forget some Third Party candidate who would win, as the only remaining candidate.   LOL

Seriously, though.
It would probably go to the House to appoint a President (the same as if nobody won the Electoral College).