Louise revels in the joy she feels at being released from the bonds of marriage and all the social roles and expectations that this bond entails. She grieves for her husband's death and she did not have a cruel thought for him personally; whether he would deserve one or not, we don't know. Louise is embracing the freedom of marriage specifically; not freedom from anything specific about Brently Mallard:
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. A kind intention or a cruel intention made the act seem no less a crime as she looked upon it in that brief moment of illumination.
Louise is not concerned with the specifics of the relationship. Whether or not she and her husband were cruel to each other is irrelevant. She is celebrating her freedom from the institution of marriage. As a married woman, she must have felt pressured to suppress her personality to fit the expected role of a dutiful wife. One can not condemn her for feeling the joy of being able to express herself and live according to her own will. She admits she will grieve again when she sees her husband's hands folded in death. Her joy is not in celebration of his death. She is celebrating the new found freedom of self-assertion that she can only feel beyond the bonds of marriage.