How are the female characters in Macbeth either frightening in their cruelty or pathetic in their helplessness?
The female characters in Macbeth are psychologically and socially complex, so I do not think that the terms "cruel" and "helpless" accurately apply to their portrayal and development. For example, one could argue that Lady Macbeth appears to be "frighteningly cruel" in the earlier acts of the play because it is she, for the most part, who sets up the murder of Duncan. When Macbeth's courage falters, she chides his manhood in order to persuade him to follow through with the murder. But if she is so cruel, why does she not commit the murder herself? She says that she would kill Duncan if he did not looks so much like her own father, so this notion at least shows a little bit of compassion. Further, later in the play, Lady Macbeth is so undone by guilt over her husband's murder of the innocent Lady Macduff that she is driven to commit suicide, which does not make Lady Macbeth appear as cruel anymore.
Similarly, one could argue that Lady Macduff is "pathetically helpless" but such an argument would neglect to take into account the historical context of the play. Set around 1050, the play positions characters for the most part in "gender-appropriate" roles. At that time, what choice did Lady Macduff have but to wait at the castle for the return of her husband?