Julia is right to mistrust Angelique -- that's how I feel at this point in our current viewing.
And while Barnabas is rightly skeptical, he does seem convinced that Angelique is genuine.
She does hesitate when Barnabas asks her if the cure is for good. Is she not sure whether her lifting the spell will be successful permanently? Or is she trying to resolve her own doubts in her own mind, not sure if she really and truly means for Barnabas to be free once and for all?
If she does mean to have set Barnabas free, how could Angelique have changed so completely?
We are prepared for this possible change of heart not once, but twice, in the series -- in 1897 and again during the Leviathan storyline, both where Angelique agrees to help Barnabas. She obviously had not changed as much as she seems to have in 1840, but the glimmer of a less selfish side to her personality was evident.
A change of heart like Angelique's is possible ... It is possible for people to grow and to learn, and I think the idea of redemption may actually be the central theme of Dark Shadows.
But at the same time, what might have happened had Angelique not
[spoiler]died[/spoiler]
at the end of the 1840 storyline? I suspect that she might eventually have gone back to her old ways. The temptations -- the jealous nature, the delight in having power over others -- may not be so easily shirked off.
Angelique redeemed -- a change of heart and mind (which is the literal meaning of "repentance") -- does not necessarily equal Angelique the saint.