Why does Mary Call decide to marry Kiser Pease? What does she think will happen?

Mary Call believes she can talk Kiser into it, no matter that she is only fourteen. Once they are married, she will make him buy the land and put it in Devola's name. Then, she will leave him and go back home. She will plant some heavy trees so she does not have to look at his house with the witch's keyhole anymore.