In The Witch of Blackbird Pond, why does Kit end her relationship with William?

After Kit is arrested and tried for witchcraft and the charges against her are dropped, William comes to call at her home. He stayed away during Mercy's illness out of consideration for the family, he says. Kit is cool toward him because he did not support her during the trial; he didn't even act as a character witness for her. When Kit shows him out that evening, they have a private conversation. Although he says he missed her, he makes it clear that he expects her to put her unpredictable ways behind her.
Hearing William's remarks, Kit realizes their relationship will never work. Although she hadn't decided previously not to marry him, the time she spent in the shed when she was under arrest gave her plenty of time to think. Now it becomes clear to her that each of them will always be wishing the other were different. She knows she doesn't care about the things that are important to him—like his house—and he doesn't care about things that are important to her—like sacrificing to help people whom others reject or ignore.
William is "baffled and unhappy" at Kit's news that she no longer wants to see him. She knows that with one word or touch, she could revive their relationship, but she lets him go. William and Kit are not compatible because they don't share the same values. Kit breaks off her relationship with William without having any other prospects for marriage. If she hadn't made that brave and principled move, she would not have been free to find a better future with Nat, who does share her values.