Haven't female murderers been treated about the same? I think it's more just people's fascinations with the motivations and feelings behind murder than some misogynistic stuff. To make that claim, you really would have to compare apples to apples. In ballads about female murderers, are the subjects treated differently? I can't really think of all that many, but in ones about Lizzie Borden, I don't recall any difference in focus or tone from the ones about male killers.