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Poor, miserable man! what right had infirmity like his to burden itself with crime? Crime is for the iron-nerved, who have their choice either to endure it, or, if it press too hard, to exert their fierce and savage strength for a good purpose, and fling it off at once! (12.2)
In other words, if you feel guilty every time you steal a paperclip from the supply closet, then a life of crime is probably not for you.
The moment that he did so, there came what seemed a tumultuous rush of new life, other life than his own, pouring like a torrent into his heart, and hurrying through all his veins, as if the mother and the child were communicating their vital warmth to his half-torpid system. The three formed an electric chain. (12.17-28)
Whew. Time for a momentary sigh of relief here as we watch Dimmesdale make this fake confession, invigorated by the idea of telling the truth of his relationship with Hester. The word "electric" strikes us as pretty fancy and somehow important. Why do you think the narrator describes the trio as an "electric chain?" (Fun fact: the electric telegraph is still really new at this point, so it's cool to think of these three as forming a telegraph line, passing messages between each other.)
To his features, as to all other objects, the meteoric light imparted a new expression; or it might well be that the physician was not careful then, as at all other times, to hide the malevolence with which he looked upon his victim. (12.33-34)
Bathed in a maybe-supernatural light, Dimmesdale's guilt is momentarily lifted, while Chillingworth just looks plain guilty. And evil.