What is Hester coming to realize is the true sin she has committed? Why would Hawthorne consider this a worse sin than her sin with Dimmesdale? In Chapter 15.

Hester considers her marriage to Chillingworth her greatest sin.

She marvelled how she could ever have been wrought upon to marry him! She deemed it her crime most to be repented of, that she had ever endured, and reciprocated, the lukewarm grasp of his hand, and had suffered the smile of her lips and eyes to mingle and melt into his own. And it seemed a fouler offence committed by Roger Chillingworth, than any which had since been done him, that, in the time when her heart knew no better, he had persuaded her to fancy herself happy by his side.