Initially, Lady Macbeth seems to be the more resolute and the more murderous of the two. Right away, when she learns of the Weird Sisters’ prophecies, she is eager to persuade Macbeth that they should murder the king and claim the “golden round” (1.5.31). She fears that he is “too full o’ th’ milk of human kindness” to take the shortest path to kingship: murdering the current king (1.5.17). Lady Macbeth, on the other hand, does not seem to be too full of kindness to casually plot murder, or to be ready to stage the murder at a moment’s notice. In fact, later, when Macbeth tries to renege on their plans, she says that she would have “dashed the brains out” of her own baby if she had sworn to do it, as Macbeth has so sworn to do this (1.7.66).
When it comes time to actually commit the murder, however, Lady Macbeth cannot bring herself to do it. She says, “Had [Duncan] not resembled / My father as he slept, I had done ‘t” (2.2.16). It turns out, then, that she does have some sentimentality or feeling in her after all. Despite his many misgivings, it is Macbeth who actually kills the king, and though he feels terrible about it after the fact, he quickly begins to plot more and worse murders. Eventually, Macbeth becomes an absolute tyrant while Lady Macbeth’s guilty conscience initiates her demise. All along, she feared it would be Macbeth who reveals what they have done to become king and queen, but it is actually she who does it in her sleep.
Macbeth and Lady Macbeth begin the play, apparently, at opposite ends of the spectrum—she is eager to murder while he is reticent, and he seems regretful while she is hardened emotionally—but as events unfold, they switch places. In the end, it is Lady Macbeth who is regretful and guilty, while Macbeth is emotionally hardened and eager to kill.