What does Hester feel is the worst crime she has committed in Hawthorne's The Scarlet Letter? Why?

In The Scarlet Letter, Hester feels tremendous guilt for having kept the identity of Roger Chillingworth secret.  For, she feels that she has been the "bane and ruin" of the man she loves.  In Chapter XVII, Hawthorne narrates,

Hester felt that the sacrifice of the clergyman's good name, and death itself, as she had already told Roger Chillingworth, would have been infinitely preferable to the alternative which she had taken upon herself to choose.

She would almost "rather lie in the half-strewn leaves and die" than confess the identity of Chillingworth, but Hester bravely tells the Reverend Dimmesdale, who responds, "I cannot forgive thee!"  However, he does forgive her, but he decries the physician who has "violated the sanctity of the human heart."