I actually think any good character will be flawed and most are, perfection becomes boring very quickly. All the characters in Wide Sargasso Sea have flaws, some are more pronounced than others, but each leads to their outcome.
Antoinette is desperate for affection after growing up being rejected by her mother. When she falls in love with Rochester she hopes that her love will finally be returned, when he rejects her it send her to the brink of insanity. Her confinement is merely a continuation of the isolation she has felt her whole life and finishes the descent into madness.
Rochester is a weak character, he marries Antoinette because of pressure from his family. He is unable to commit himself fully to Antoinette and seeks out reasons to doubt her. When he is given the opportunity, he chooses to reject Antoinette. From enotes, "throughout his narration in Part Two, Rochester presents himself as a helpless victim of circumstance. In his bitterness, he lashes out at Antoinette, spurning her love and destroying her potential for happiness as well as his own".