Now, excluding for the time being the circumstantial evidence against Ronald--though without losing sight of it--the next point that arises is was he murdered by somebody in the inn or by somebody from outside--say, for example, one of the villagers employed on his excavation works.

The waiter's story of the missing knife suggests the former theory, but I do not regard that evidence as incontrovertible.
The knife might have been stolen from the kitchen by a man who had been drinking at the bar; indeed, until we have recovered the weapon it is not even established that this was the knife with which the murder was committed.

It might have been some other knife.

We must not take the waiter's story for granted until we have recovered the knife, and not necessarily then.

But that story, as it stands, inclines to support the theory that the murder was committed by somebody in the inn.