Why does Lady Macbeth tell that these deeds must be thought of?

Lady Macbeth says that the murder of Duncan must be thought of because she believes that it is necessary for her husband, Macbeth, to become king. She argues that if they do not act now, they will lose the opportunity to seize power and that Macbeth will forever be haunted by his ambition. She also believes that they can get away with the murder if they plan it carefully and do not let their emotions get the best of them.