She locks her husband and Jennie out of the room for all of those reasons.
She is attempting to free the woman from behind the wallpaper (and herself from the constraints of her husband's cruelty). In order to free the woman, she has to be able to remove the wallpaper. Since, as the narrator describes the situation, it's fairly clear that she's going insane, she believes that she has to get all the wallpaper off so that they can't put the woman (her) back into the wall. It's important to the narrator that they not stop her from removing the paper.