What reason does lady Macbeth give for not killing Duncan herself in Macbeth?

In Act II, scene i of Macbeth, Lady Macbeth
admits:

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Had he not
resembled
My father as he slept, I had
done't.

In
Act I, Lady Macbeth's speech resembled that of a man's: she cast of her femininity and devised a
brilliant plan of murder.  But here in Act II, her actions cannot follow through on her earlier
boasts.  She can motivate Macbeth to kill, but she cannot kill herself.  Murder is a man's
business.

Lady Macbeth becomes too emotional during
the attempted murder: Duncan reminds her of her father. She sees him as a real
person, and the sight of blood unnerves her, producing immediate guilt.  Later, her guilt will
intensify and turn to mania: she will incessantly try to wash off blood from her hands that is
not literally there.

Here, Lady Macbeth suffers from a kind of
Elektra Complex (the female version of the Oedipus Complex).  She can kill off the feminine,
mother-figure in her, but she cannot kill the opposite-sex parent-figure, Duncan, because she has
an unconscious attachment to him.