The answer to this question necessarily involves an understanding of gender and the way that women were thought of and treated at the time when the novel is set. Women were thought of as being fragile and weak and incapable of committing crimes such as murder. Critics have noted the different treatment that Grace Marks receives from the law compared to James McDermott, who was quickly found to be guilty and hanged for murder. There was no discussion about his guilt or otherwise, and some argue that this is because of gendered assumptions about what men and women can't do. Grace herself recognises that being branded as a murderess is something that is oppressive, but what she finds attractive about it is that there is a mystery, an allure, that causes other people to treat her differently. Note what the quote says:

It has a smell to it, that word--musky and oppressive, like dead flowers in a vase. Sometimes at night I whisper it over to myself: Murderess, Murderess. It rustles, like a taffeta skirt across the floor.

Murderer is merely brutal. It's like a hammer, or a lump of metal. I would rather be a murderess than a murderer, if those are the only choices.

Grace decides that she would rather be called a "murderess" than a "murderer" because of the associations that go with that word. To be a "murderer" is to be "merely brutal," but to be a murderess, although it brings its own challenges, is infinitely preferable, because of the way that being a "murderess" goes against expected gender norms of the day and therefore gives that person a kind of celebrity status.