why does mrs. wright kills her husband in trifles?

Mrs. Wright has been kept away from the rest of the community by her husband. She resents the fact that she's not allowed to socialize or even attend functions at the church. When Mr. Wright becomes angry at her and kills the canary, he is in essence killing killing his wife's spirit. The canary was symbolic of what Mrs. Wright was like before her marriage; happy, singing, wearing colorful clothes, and yet the canary's existence in its cage symbolizes her life since marriage.

When he kills the canary, Mrs. Wright goes over the edge emotionally. The broken cage and the sight of the little bird with its broken neck was more than she could bear. She ended her husband's life in the same way he ended the canary's........... and rather in the way he's ended hers, by suffocation.