Why does Mrs. Walker's life end tragically in "The Devil and Tom Walker"?

Tom's wife certainly does get more than she bargained for when she goes into the forest to do business with Old Scratch. All that Tom finds left of her is her heart and liver tied up in her apron, hanging from a tree with a vulture hovering over it. Her gruesome fate contributes to the story in several ways.

First, "The Devil and Tom Walker" is an example of Romanticism in literature. Mystery, death, and strangeness are all elements of Romantic literature. Tom's wife's dying in such a strange and mysterious way contributes to the story's strong Romantic tone.

The horrible nature of her death also contributes to the theme of the story and to Tom's character development. She is a very, very greedy woman; she is so greedy she will deal with the Devil himself if it will enrich her. For her to die in such a shocking way emphasizes the terrible results of greed. Also, Tom's reaction when he finds what's left of his wife's body shows a great deal about him. He is only upset because he has lost his property. Finding her heart and liver doesn't seem to faze him, but he does notice the signs of a fierce battle. He assumes she put up quite a fight:

Tom knew his wife's prowess by experience. He shrugged his shoulders, as he looked at the signs of a fierce clapperclawing. "Egad," said he to himself. "Old Scratch must have had a tough time of it!"

Tom clearly did not love his wife! In fact, he was glad she was dead:

Tom consoled himself for the loss of his property with the loss of his wife . . . .

Tom is so hard and cold emotionally that even the horrible way his wife dies does not move him at all. Like her, all he loves is money.

Why does Mrs. Walker's life end tragically in "The Devil and Tom Walker"?

The setup of the story seems to lead us to predict that something bad will happen to Tom and his wife. The treasure is clearly established as being associated with the Devil, and that the time period in which the story takes place was occasioned by many earthquakes, which at the time were often taken as religious signs of unrest. The fact that the treasure is introduced at all will lead us to expect that someone is going to find it, and receive the mixed blessing of its wealth and its negative associations. 

When we are introduced to Tom and his wife, they appear as one-dimensional and petty people who are ruled by basic vices and feelings. An insightful reader might predict that they are doomed to encounter the treasure and have their lives changed by it, and the fact that this is how the story plays out may make it less than satisfying, since the characters are simply meeting our expectations at almost every turn. On the other hand, seeing blatant evil being blatantly punished gives the story an arc and may explain why so many other fables have a similar structure.

There are at least two reasons for Tom's wife being dismembered (he finds only her heart and liver tied up in an apron). One is to demonstrate the Devil's destructive power, which until that point had only been implied. The other is that his wife directly picked a fight with the Devil, as evidenced by tufts of black hair on the ground; it seems that she thought she could fight with the Devil just as she did with her husband. Another reason may be that she was simply less important to the story, and killing her allowed both the Devil and Tom to be further characterized by her death, as well as eliminating her from the story to allow Tom further freedom of action.

What are Mrs. Walker's emotions in "The Devil and Tom Walker"?

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.