First, her sin is committed out of love and not from any negative motive, and second, her quietness and industriousness afforded her the time for self-reflection that led to her change and salvation.
Abigail, in contrast has no family in the Crucible. She latches on to John Proctor as a sort of father/husband substitute, reflecting the twisted nature of her manipulation of the other girls also reflects the lack of early family relationships. The culture of Salem in the play is also different from Boston in the Scarlet Letter; the people are far more selfish and greedy, and the availability of land creates a very different dynamic amongst the people that fosters Abigail's evil tendencies. At heart, however, Abigail is simply a self-centered and shortsighted girl. These two personal attributes are the main cause of her downfall, just as Hester's innate goodness is the main source of her salvation.
Miller focuses a created, heterosexual alliance in his fictional retelling, but I, Tituba concentrates on the outcasts, which formed the actual, majority of the accused. This alliance between marginal categories of persons is humorously underlined with Tituba meets a famous fictional outcast from Puritan society, Hester Prynne, while in jail. Conde creates a jailhouse meeting between the two women, since who knows what transpired while Tituba awaited her fate? Marginal