Many people do argue that Lady Macbeth is the more villainous of the two because Macbeth attempted to call off their traitorous, murderous plan. In Act 1, scene 7, he tells her, "We will proceed no further in this business" (1.7.32). He has considered the myriad reasons he has not to kill Duncan: Duncan is his guest, his king, his friend, his kinsman -- Duncan is a good king and a good man. Macbeth admits to having only one reason to commit the murder, his "Vaulting ambition," which is, evidently, not enough to compel him to go on with the plan. It is not until Lady Macbeth insults his bravery and his masculinity that he relents and recommits to kill his friend. She goads him on, pushing his buttons and deftly manipulating him so that he does exactly as she wants.
However, Lady Macbeth later displays much more of a conscience than Macbeth does. After he's ordered the murders of Banquo, Fleance, and Lady Macduff and her innocent children and servants, Lady Macbeth is seen sleepwalking, imagining that she cannot clean the king's blood from her hands. She, at least, feels guilty, so guilty that she becomes unable to function and eventually takes her own life. Macbeth grows worse and worse, more unscrupulous by the day, and some would use this as evidence to argue that he is the worse of the two.