I repeat: Duty to what ?" And, rising, he went over to the window, looking out at the moonlit lawn, overcome by a sudden aversion from more talk.

Of what use were words from a mind tuned in one key to a mind tuned in another?
And yet, so ingrained was his habit of discussion, that he promptly went on: "The Mallorings, I've not the slightest doubt, believe it their duty to look after the morals of those who live on their property.

There are three things to be said about that: One--you can't make people moral by adopting the attitude of the schoolmaster.

Two--it implies that they consider themselves more moral than their neighbors.

Three--it's a theory so convenient to their security that they would be exceptionally good people if they did not adopt it; but, from your account, they are not so much exceptionally as just typically good people.