Lady Macbeth is much more the serpent. In Act I, scene five, she pleads with the spirits to strip away her femininity:
"Come, you spirits
That tend on mortal thoughts, unsex me here
And fill me, from the crown to the toe, top-full
Of direst cruelty!" (I.v.41-44)
If Lady Macbeth was more like a flower, she certainly would not desire the spirits' involvement in tearing away her delicate virtues which would be much more flower-like qualities. Her motives are much more serpent-like; she exhibits cunning and craftiness, which are common character traits associated with serpents. Similar to the serpent and Eve in the garden of Eden, Lady Macbeth has felt temptation and now desires power and the resolve to commit whatever acts necessary to achieve it. There may be no 'forbidden fruit' here, but the act of murdering King Duncan is treasonous. Lady Macbeth embraces the opportunity to heighten her husband's status. Later in the same scene, she instructs Macbeth:
"Look like the time; bear welcome in your eye,
Your hand, your tongue; look like the innocent flower,(70)
But be the serpent under't" (I.v.68-71).
She heeds her own advice, certainly proving her own ability to "be the serpent," proving herself a charming hostess to the king while steeling herself to aid in his murder.