How does Hester interpreth the changes she sees in Chillingworth?

Chillingworth is always described as older, rather weathered man to an evil incarnate and twisted old man. Check out their quote,

"But the former aspect of an intellectual and studious man ... had altogether vanished, and been succeeded by a eager, searching, almost fierce, yet carefully guarded look. 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. 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..."

Hester notices the disturbing changes. She begins to understand his creepy hold on Dimmesdale and the extent that he will go to control him. Hester believes this goes beyond simply punishing the man who committed adultery with his wife; he needs to control him as part of his own identity.