The narrator immediately remarks that Mrs. Mallard had heart trouble and this was why the news of her husband's (alleged) death was broken to her gently. This is logical but it also supposes that Mrs. Mallard is mentally as well as physically weak and/or incapable of dealing with traumatic news in a rational way. On the contrary, after grieving dramatically for a spell, Mrs. Mallard actually becomes stronger. Thus, once thought a weak, dependent woman, Mrs. Mallard had actually found liberation following her husband's death.
Upon hearing of Brently's death, Mrs. Mallard (Louise) "wept at once, with sudden, wild abandonment, in her sister's arms." After this dramatic grieving, Louise goes to her room alone, sobbing quietly and occasionally in her armchair. About this time, her transformation occurs. (Note the terms "repression" and then "strength.")
She was young, with a fair, calm face, whose lines bespoke repression and even a certain strength.
Then, realizing that she had been freed from her role as the wife, she began to repeat the word "free." And note that it is clear that she and Brently had a good marriage. So, it was not the case that Louise awakened from an abusive or unloving marriage. It was the case that she had awakened and escaped from the role that she was forced to play as a dutiful wife, often relegated to the home, as was the custom for women in the late 19th century. So, she had been liberated from that role. Now, no longer suppressed, her life belonged to her; rather than to her husband:
There would be no one to live for her during those coming years; she would live for herself. There would be no powerful will bending hers in that blind persistence with which men and women believe they have a right to impose a private will upon a fellow-creature.
An escape from an unloving, abusive marriage would certainly be a liberation. However, looking at this story through a feminist lens, Louise's escape is a liberation as well because she is no longer tethered to the will of her husband and no longer forced into that wifely role which carries much less freedom than the role of the husband does. Louise's liberation is also a realization for her; she realizes that up to that point, she had been a prisoner of sorts, albeit it in a comfortable marriage. Now with her husband gone, she feels truly free: "she was drinking in a very elixir of life through that open window." The shock at seeing Brently is described by the doctors as a "joy that kills" but it might certainly have been the loss of joy that killed since her new found freedom was taken from her at this moment when she was fully realizing it.