The main thematic concern of the play is patriarchy's devaluing of women.
Patriarchal abuse is shown through what happens to Minnie Wright. She has had her spirit crushed by her grim husband, who won't let her spend money, as Mrs. Hale realizes from the state of Minnie's clothes. He also doesn't treat her kindly. Minnie, crushed and isolated, turns to her pet canary for solace. When her husband kills the beloved bird, an example of his cruelty, that becomes the final straw. Minnie snaps and kills her husband.
The play makes the point, however, that this is more than an isolated incident. The way all the males in the play treat women is condescending and belittling. The males investigating John Wright's death are dismissive towards Mrs. Hale and Mrs. Peters, who have come with them to the crime scene to get clothes for Minnie. They put down what the women say as merely being concerned with "trifles" of no consequence. They underestimate and laugh at the contribution women's work makes to their lives.
As a result of their inability to see the value of the domestic arena they find trifling, the men completely miss the evidence the women see. Further, because the men are so belittling toward Mrs. Hale and Mrs. Peters, the two won't share what information they have. They feel far more protective toward Minnie than they do toward a legal system that is run by men and unlikely to understand her plight.
Through the behavior of the men at the farmhouse, the play makes the point that the widespread acceptance of treating women as second class allows crueler men like John Wright to get away with oppressing their wives and destroying their spirits.