In The Scarlet Letter, what change did Hester see in Chillingworth's face after seven years?

In chapter 14 of The Scarlet Letter, titled "Hester and the Physician", Hester goes to Chillingworth and wants to have a word with him. Chillingworth starts to talk by telling Hester about how the magistrate has been debating over the use of the scarlet letter, and whether Hester should continue to wear it. After a brief discussion on the matter, Hester looks deep into the face of the man, and notices great changes. She

...was shocked.... It was not so much that he had grown older... But the former aspect of an intellectual and studious man, calm and quiet...had altogether vanished, and been succeeded by an eager, searching, almost fierce, yet carefully guarded look.

Like the passage states, Chillingworth is not necessarily a faded-looking old man. He is aging, of course, but the chapter even says that he "bore his age well," had a certain amount of "vigor," and was altogether quite alert.  The problem is that his anger and hatred are essentially coming through his facial expression, and he can no longer hide them. What Chillingworth has against Dimmesdale is an obsession with vengeance that he himself can't rationalize or justify. As a result, this hatred consumes even his physical appearance.

Chillingworth cannot quite conceal this angry, wild look that Hester notices. Even when he tries to smile, the fake nature of the smile comes through, to the point that his "blackness" is more evident than anything else. Other things were indicative of a man so angry and consumed by hatred that he can no longer control himself:

Ever and anon, too, there came a glare of red light out of his eyes; as if the old man's soul were on fire, and kept on smoldering ...until..it was blown into a momentary flame. This he repressed as speedily as possible, and strove to look as if nothing of the kind had happened.

So what we have is a facial expression where the eyes glare with anger, and the gesture is altogether wild, while "carefully guarded." The man is consistently watchful of concealing his anger, and thus he fails at it because he feels too much of it.  

The key difference is that Chillingworth was once a passive, pensive, and quiet man. Those were the key traits that Hester remembers most about him. Now, those traits are gone and she is the one who can see this difference as the woman who was once his wife.