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Mistress Hibbons practices witchcraft, she is the bigger threat to society, but she also represents the society's acceptance of evil. What she does isn't a secret, and she understands that she'll eventually be executed, but Hester is so much easier to punish.
Mistress Hibbens actually participates in witchcraft and witch rituals at night, thus practicing what the Puritans would have thought of as the deadliest of evils. Yet, she goes unpunished even when Hester, whose sin was so obviously shown, letting the reader know that there is a great deal of hypocrisy in the Puritan society. Both Hester and Mistress Hibbens represent a threat to the social stability because both violate the very strict rules of the Puritans, yet Mistress Hibbens may be a bigger threat simply because she has not yet "faced the music" of the Puritan society, thus showing the illogical thinking these people have.