In chapter III of Nathaniel Hawthorne's The Scarlet Letter we find Hester Prynne standing at the scaffold going through the humiliation of being publically exposed and questioned by the elders of her settlement as a result of her indiscretion: The fact that she becomes pregnant from a man whom she refuses to identify, and what is worse, she is pregnant out of wedlock.
We are not aware of the exact gravity of her indiscretion until we realize that Hester is not a widow as she thinks: Someone has just entered the settlement and is lurking from within the crowds looking specifically for her. It is her husband. He is described quite characteristically:
He was small in stature, with a furrowed visage, which, as yet, could hardly be termed aged. There was a remarkable intelligence in his features, as of a person who had so cultivated his mental part that it could not fail to mould the physical to itself, and become manifest by unmistakable tokens. Although, by a seemingly careless arrangement of his heterogeneous garb, he had endeavoured to conceal or abate the peculiarity, it was sufficiently evident to Hester Prynne, that one of this man's shoulders rose higher than the other.
Robert Chillingworth is the new name Hester Prynne's husband has donned himself as a way to remain unknown to the people of Boston. He has suffered the humilliation of finding out that his wife has committed a huge faux pas and now he is determined to take revenge. Up until the moment she stands at the scaffold, Hester has no idea that Chillingworth has entered the town. However, the sight of him is certainly terrifying:
...at the first instant of perceiving that thin visage, and the slight deformity of the figure, she pressed her infant to her bosom, with so convulsive a force that the poor babe uttered another cry of pain. But the mother did not seem to hear it.
Therefore, the sight of her husband, who she thinks is dead and who is now facing her, and her sin, become the single most terrifying moment in Hester's life. So terrifying indeed that she "convulsively" feels the need to protect her child. This shows that, deep inside, she is aware of what he is capable of doing.