What a terrible accident. Condolences to those who have lost loved ones.
For the injured, deepest sympathy.
There are significant clues in the pictures and videos that are already here.
When you fly a loop and you start at ground level, which is what happend here, you must bank some excess airspeed to ensure you will have some energy left for the pullout in a maneuver which is essentially an energy consuming maneuver in a non-afterburning jet.
This maneuver required converting that excess energy into altitude, but that did not happen here. Shortly after the 90 degree pitch attitude, the aircraft rolled off heading and then pulled quickly to the nearest horizon. When the pilot elected to continue the maneuver from there, that created the accident.
It is very likely that the pilot experienced either a gray out or GLOC event during the climb phase. That would be a reasonable explanation for the strange events leading to the top of the loop.
Ive been burnt a couple of times while flying in airshows, and have subsequently refused to volunteer. You have to be completely on top of your game for them, and even then the fickle finger of fate can tap you on the shoulder.
The carnage in this accident is a sad reminder that aviation is actually a rather dangerous art that requires extreme levels of planning and execution to produce a safe result. Accidents like this one take the fun out of aviation.