At the banquet that night, Lady Macbeth does her part by flattering Duncan in any way she can, which works to disarm him. Later, when Macbeth is beginning to have second thoughts about murdering Duncan, as he enjoys being in favor with the king, Lady Macbeth steels his resolve. She reminds him of his promise to kill Duncan and insists he will be coward if he backs out. She says to him that she would go so far as to dash her own baby's brains out if she had promised Macbeth to do so. She also reassures him that the murder will succeed.

 

Thus, from the start, we see that Macbeth and his wife are solidly in league and committed to each other in their murderous enterprise.