The Pale Horse - Agatha Christie

In order to understand the strange goings-on at the Pale Horse Inn, Mark Easterbrook knows he must go back to the beginning...but where exactly do those beginnings lie?

Was it the brutal blow on the head that Father Gorman received? Or in his visit - a few minutes earlier - to a woman on her deathbed? Or is it in that violence that Mark himself witnessed sometime before that?