what reason does Lady Macbeth give for not killing Duncun Herself

From the text:

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

Lady Macbeth's inability stems from her femininity. She can cast away her womanliness to provoke her husband to murder, but she cannot bring herself to kill Duncan...... murder is a man's business, and thus, she leaves it to Macbeth.