In Act 2, Scene 2, Macbeth returns from murdering Duncan, but he has brought the grooms' two daggers with him. This seems strange. Apparently he is either in a trance or else he has forgotten that the plan was to smear the faces of the sleeping grooms with blood and leave the daggers with them so that they would be presumed guilty of the crime. Lady Macbeth tells her husband he must take the daggers back to Duncan's chamber, but he refuses. He says he doesn't dare to look at what he has done. This is where she says:
The sleeping and the dead
Are but as pictures; ’tis the eye of childhood
That fears a painted devil.
She is comparing the dead king with the sort of scary pictures, dummies, and wooden dolls that are brought out on occasions like Halloween. They are intended to be frightening, but only little children are ever frightened. So she is saying that her husband is acting childishly. The sleeping and the dead are but as pictures, and the painted devils are also only pictures. There is obviously a difference in their characters. She is a realist. She can look at the dead king without feeling frightened because she knows he is like a picture; whereas Macbeth is not frightened at the prospect of merely looking at Duncan but horrified at the prospect of being reminded of what a truly awful thing he has done. Duncan may be dead, but there is still a lot more to come--which Lady Macbeth doesn't even think about. She will be relieved of a lot of that stress because she is a woman. As Macduff says in Act 2, Scene 3:
O gentle lady,
’Tis not for you to hear what I can speak:
The repetition in a woman's ear
Would murder as it fell.
Macbeth will be heavily involved in the aftermath of this momentous event. The body will have to be discovered. There will be a great uproar and then an inquiry. Macbeth will have to pretend to be as horrified and as outraged as everyone else. The worst is yet to come. And he is hoping that somehow this murder will lead to his being made king, as the witches seem to have promised. But even if he is made king, there will be plenty of people who will suspect him of having, as Banquo says at the opening of Act 3, "play'dst most foully for't." Macbeth will have to live with his guilty conscience for the rest of his life; whereas his wife seems to be capable of wiping it out of her memory. But we learn late in the play that she lacked her husband's intelligence, imagination, and foresight. She was not capable of wiping the deed out of her memory, as we see in her famous sleepwalking scene in Act 5, Scene 1. Throughout these early scenes just before and after Duncan's murder, Shakespeare is attributing to Lady Macbeth the lack of ability to think in abstractions which he apparently attributes to women in general.