To answer this question, we first have to figure out what the ending isn't. It is certainly true that Wide Sargasso Sea is a kind of prequel to Charlotte Brontë's Jane Eyre. (For more on the Jane Eyre connection, see "In A Nutshell.") But we can't take for granted that what holds true for Jane Eyre holds true for Wide Sargasso Sea as well. Thus, while in Jane Eyre, Antoinette (called Bertha in Jane Eyre) sets Thornfield Hall on fire and leaps to her death, in Wide Sargasso Sea we never actually see Antoinette doing any of this except in her dream. In fact, the novel's last lines are ambiguous:
Now at last I know why I was brought here and what I have to do. There must have been a draught for the flame flickered and I thought it was out. But I shielded it with my hand and it burned up again to light me along the dark passage.
It is certainly possible that Antoinette will set fire to Thornfield Hall, but this becomes only one possibility among others. For "passage," the very last word of the novel, asks us to consider all of the different ways that Antoinette is passing from one state to another – physically, certainly, but psychologically, culturally, even politically as well.
Does Antoinette's appeal to black characters such as Tia and Christophine in her dream imply that she rejects her white Creole identity for a black Caribbean one, symbolized by her setting the house on fire just as the black rioters did to Coulibri earlier in the novel? Then why not show her actually doing this? Why keep it in a dream?
Or is the "passage" a passage into selfhood, a way of recovering from the psychological trauma that troubled her from childhood and into adulthood with her relationship with Rochester? As Part III progresses, she transitions from having no knowledge of who she is to remembering everything. Thus the fact that she has dreamed her dream for the last time, a dream that incorporates scenes from her entire life, could be a way of overcoming a life-long, self-destructive pattern of behavior.
Or is the passage a literary one, a passage that signals Antoinette's final emergence from behind the shadowy fiction of Bertha Mason in Jane Eyre? With this view, Antoinette is neither a political heroine or psychologically cured, but just a dramatic representation of how voices are silenced in the great texts of English literature.
These are just a few of the possible outcomes, so go ahead – get lost in the novel for a while and see what you come up with. Appropriately for a novel that is itself a re-reading of a literary classic, the ending of Wide Sargasso Sea invites endless re-readings and interpretations.