What reason does lady Macbeth give for not killing Duncan herself in Macbeth?
In Act II, scene i of Macbeth, Lady Macbeth admits:
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.
The reason that Lady Macbeth gives for not killing Duncan herself is the fact that she is a woman. Lady Macbeth was a very ambitious person for herself and her husband, however, it was her husband (according to what is expected of her time and place in society) that should commit the act. Therefore, Lady Macbeth questioned her husband manhood when she suggested this and may have used this as a tactical point to encourage him to commit the act.
It would be a type of Shakespearian reversed psychology, like for example when a woman tries to convince a man to do something that takes a lot of (literally) testosterone to dare to do and then when the man reasons that this is not the way, the woman decides that she may have the testosternone for the both of them together and, if she were a man, she would do it herself.
It's a bit of a castrating message from a wife to a husband not to mention the severity of it.