Mrs. Walker is a fairly one-dimensional character, meaning that she exhibits very little depth, or variation, beyond a certain series of behaviors.
She is introduced as a miserly person, perhaps moreso than Tom, and all of her actions seem to be underlain, defined and inspired by greed. In this sense she seems a bit unrealistic, or at least exaggerated for the purpose of the story.
She is frequently described as a "termagant", an archaic term for a loud and overbearing woman. This, combined with the details of her constant fights with Tom, suggest that the emotions she feels most often are anger, frustration and jealousy. These are ultimately the cause of her demise as well.
Mrs. Walker becomes blatantly jealous at the thought of Tom not taking the opportunity to strike the bargain with Old Scratch, and determines to make the deal and take the treasure on her own. As evidenced by the footprints and tufts of hair that Tom finds near her remains, it appears as though she initiated a physical fight with the Devil, presumably because she thought she could "negotiate" with him in the same way that she did with Tom (by hitting him). Thus her death was most likely caused by her limited emotional range and short temper.