Minnie Wright felt trapped in her marriage. Through dialogue, the women discuss how she seemed unhappy and rarely went out; they didn't see much of her because her husband kept her confined to work the farm. It seems that she was a much more extroverted person before the marriage; Mrs. Hale mentions that Minnie used to sing, and that her marriage "killed" that aspect of her. The women discover the dead body of her canary, which would sing, and surmise that her husband killing the canary was the last straw:
MRS. PETERS: [With rising voice.] We don't know who killed him. We don't know.
MRS. HALE: [Her own feeling not interrupted.]If there'd been years and years of nothing, then a bird to sing to you, it would be awful -- still, after the bird was still.
(Glaspell, Trifles, etext.virginia.edu)
It would have been the constant piling of miseries that pushed Minnie to murder. Without any outlet for her personal frustrations, and without the ability to express herself emotionally or creatively, she felt that her only recourse was to eliminate the negative influence of her husband. The canary, which was probably the only nice thing in her life, represented her link to sanity and also to acceptance of her life; when it was killed, she refused to accept her misery any longer and took matters into her own hands.