Once Macbeth becomes king, instead of enjoying his new status, he begins to fret about what the witches have prophesied for Banquo. Macbeth asks himself if he has killed Duncan so that Banquo's descendants can become king and states, "There is none but he whose being I do fear." As a result he decides to have his friend Banquo and Banquo's son murdered, but for this murder Macbeth does not consult Lady Macbeth. From this point on in the play, they are no longer "partners in greatness."
Macbeth will not be satisfied with the death of Banquo because after he visits the witches again, his fears about Macduff are confirmed--although the Weird Sisters assure him that "none of woman born can harm Macbeth." To lure Macduff back from England, Macbeth orders that everyone in Macduff's family and in fact everyone in the whole castle be slaughtered. These murders are even worse than killing Banquo and Duncan because innocent women and children die. Macbeth has become a tyrannical monster who is acting entirely alone without any influence from Lady Macbeth.