Minnie killed her husband because he was abusive toward her. She killed him in self defense.
There is plenty of evidence that Minnie was abused. She used to be a cheerful, outgoing, and carefree young girl before she got married. Then she married and everything changed. The dead bird is a perfect metaphor for what happened to her. There is no evidence of physical abuse, but there might have been. There is plenty of evidence that he was controlling, and emotionally abusive toward her.
Although Minnie killed her husband, there were mitigating circumstances. Mrs. Hale reflects on what Minnie was like before she was married.
I wish you’d seen Minnie Foster when she wore a white dress with blue ribbons and stood up there in the choir and sang. … Oh, I wish I’d come over here once in a while! That was a crime!
Mrs. Hale blames herself. She is aware that Mrs. Foster was being abused and tortured by her husband, and was basically abandoned by her so-called friends. No one was aware of what was happening to this poor woman. They noticed the change in her, but they didn’t see the “signs.” They didn’t realize that she was being emotionally tortured. Mrs. Peters did not come and visit. She did not intervene. She did not come to save her.
When they notice that there was a bird, it is a metaphor for Minnie. They realize that Minnie was like a bird. She was delicate and used to sing like a bird socially. When she got married, she no longer “sang.” When they finally find the bird, they realize someone has wrung its neck. It was obviously her husband. He killed her bird literally, and killed her figuratively. It is then that the women realize the depths of her Hell.
(Their eyes meet. A look of growing comprehension of horror. Steps are heard outside. Mrs. Hale slips box under quilt pieces, and sinks into her chair. Enter Sheriff and County Attorney. Mrs. Peters rises.)
That poor woman! Was Minnie justified in killing her husband? It was the only way out? Perhaps he was about to wring her neck, just like he did that bird. It seems just as likely that the man who tortured her, and took her from being a bright and carefree young woman to a timid and sad one, was about to take her life when she decided to take his. We can see all kinds of signs of distress in this house.
The signs seem like trifles to the men-- the broken jars, a dead bird, and unused quilt pieces in a box—but the women see them for what they are. They are signs of a women in trouble, and no one helping her until she did what she had to do, and killed her husband in self-defense. Even Minnie sitting in the rocking chair, which the men take as signs of her guilt, is just another sign of her abuse and the fact that she killed him in self-defense and is in shock.