In chapter 14, titled "Hester and the Physician" we find Hester and Chillingworth finally getting together up close and almost personal. It is here, when Hester has the best opportunities to observe and see what her former husband has become in the last seven years:
It was not so much that he had grown older[...]But the former aspect of an intellectual and studious man[...] had altogether vanished, and been succeeded by an eager, searching, almost fierce, yet carefully guarded look.
Here, we see that Chillingworth is not necessarily a scraggy, old-looking man. It is not the actual age that has worn him down, but that fastidious need to find out what is going on with Dimmesdale, his own personal vendetta against Hester, the hurt and broken ego, and his desire to cause chaos. One can imagine that the man was hurt badly by his wife, unbeknownst to her at the time. Yet, we also tend to sympathize with Hester and find his want for justice just that: a bit fastidious.
It seemed to be his wish and purpose to mask this expression with a smile; but the latter played him false, and flickered over his visage so derisively, that the spectator could see his blackness all the better for it.
This quote, same chapter, shows that all the good that could have come out of this intellectual, gifted man has turned into hatred and evil- all caused by the same anger and want for revenge that does not seem to leave his spirit. Chillingworth's transformation of the soul is now manifesting in his façade. This is not a physical change, but a spiritual one, and a bad one at that.
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 smouldering duskily within his breast, until, by some casual puff of passion, it was blown into a momentary flame.
This quote shows his emotional failures and changes. Chillingworth is, indeed, heartbroken. There is no doubt that he loves Hester still, and he knew from the beginning that he would never serve her effectively as a husband. He even goes as far as telling Hester that he thought that his intellect would compensate physical vigor, passion, or that it may spark interest in Hester one day. None of it ever happened, and it is clear that he has repressed his emotions to overcompensate his revenge.
These changes even scare the man, himself. At one point he is angry at Dimmesdale because, supposedly, Dimmesdale made a demon out of him. It is here, where Chillingworth takes a sudden glimpse of his own image...and it scares him!
The unfortunate physician, while uttering these words, lifted his hands with a look of horror, as if he had beheld some frightful shape, which he could not recognize, usurping the place of his own image in a glass.
Therefore, you can see that the man is directly and indirectly characterized as someone who has undergone profound changes that stem from something very negative and angry that has the capacity of transforming him forever.