The people responsible were, surely, the driver Dodi for not telling him to slow down and for not ensuring seat belts were worn, Diana herself for not wearing hers, and the paparazzi for chasing them so fast.
It is inconceivable that the entire French health system, police and security forces would connive with the British Royal Family and security forces to murder Diana, and especially not in a way which left so much to chance.
If you look at the Wikipedia article it deals with most of the theories. For example, the reason the ambulance passed other hospitals was because it was heading for the one with the specialist knowledge of trauma injuries.