The beauty of the ending to the novel is that the reader is left to assess Edna's actions. In my own reaction to it, I sense that Edna's distillation and understanding of what it meant to be free led to this particular point. She had spent the majority of the novel seeking to better understand who she is and in what she believes. This leads to an emotional and psychological "awakening," one in which Edna is able to critically assess her own life and her own way of being. This resolution is one in which Edna realizes that her own sense of identity will always be kept dormant in a social order that thrives on conformity and ensuring that individuals are not "awakened." For Edna, this becomes especially so. The forces that sought to shackle her, ones that she perceived to be elements to repress, are mere voices as she enters the water. In seeing the bird with a broken wing fall, I sense that Edna realizes that she is not going to be broken. Her act of entering the water, completely naked, is something that I see as a form of defiance, a way to maintain that her awakening will not be vitiated by society. I interpret her ending as one of complete resistance and defiance, the ultimate act of one who has "awoken" against a society that is still asleep.
I do not agree with Edna's decision at the end of the novel. I feel that even though she wanted to be her own person and get away from her life, that is not the way she should have done it. Her last thoughts were of her husband, children, and Robert. I feel as if what pushed her to kill herself was that she realized her responsibilities as a mother.
Reminder: This is all my opinion.