He discovered that she hated to hit him on the hand with a ruler (he scarcely felt the strokes).

He discovered that when his mother or father was in the room she was terrified lest he should misbehave.
He discovered that she was despised by the servants, who quite openly insulted her.
All these things fed his sense of power.

He did not consider her a human being at all; she was simply something upon which he could exercise his ingenuity and cleverness.

Mary followed him in whatever he did; Helen pretended to be superior, but was not.

Yes, Miss Jones was in the hands of her tormentors, and there was no escape for her.
Surely it must have been some outside power that drove Jeremy on.