Lady Macbeth, in Shakespeare's Macbeth, tells her husband that if she broke a promise like he is thinking of breaking a promise, she would pull a baby from her breast while breast feeding, and smash it.
The promise Macbeth is thinking about breaking concerns the assassination of a fair and noble king. And the promise itself, was never made.
Macbeth's only commitment to assassinating Duncan is a statement that they will speak again of the matter. He never swears to do it, as Lady Macbeth implies. She manipulates the facts and her husband in order to get him to kill the king.
She also, of course, drugs the grooms so that Duncan can be killed, and helps with the clean up and the cover up. She is also the one who plans the assassination. In short, Lady Macbeth longs to be savage and merciless like a male warrior, and is as ambitious as any man. She is willing to kill a ruler who is considered to rule according to divine right, in order to make her husband king and herself queen.
At the same time, you should be aware that Lady Macbeth actually does very little physically. She is unable to kill Duncan herself because he reminds her of her father--quite sentimental for an "evil" person. And when Banquo and Macduff's family are murdered, she ultimately suffers a mental breakdown. She engineers Duncan's murder, but she never planned for the murder of wives and children. Macbeth plans those killings on his own.
By your question, you seem to assume that Lady Macbeth is evil. I would contend that rather than being evil, Lady Macbeth is a human being who succumbs to evil.
In Act I, scene 5, Lady Macbeth reads her husband's letter informing him of the prophesies of the Weird Sisters. She loves her husband deeply and feels that he would be a better king than either Duncan or Malcolm. She also know that he does not have the temperament to actually kill the king.
News that the king will be spending the night seem to her the perfect opportunity to do what she feels must be done. Yet, she is not strong enough to do it either. If she were really evil, she would not have to call upon, "...you spirits/That tend on mortal thoughts..." aka evil spirits to unsex her and fill her with evil. She knows that as a woman, her feminine nature is naturally against such actions. Question, is her prayer answered or does she just think that it is answered?
If she was truly evil, why doesn't she kill him herself? In Act II, scene 2, she tells Macbeth that she has drugged the grooms and laid the knives ready for him to use. She thensays, "...Had he not resembled/My father as he slept, I had done't." I would imagine that evil would be made of sterner stuff than that. A truly evil person would not have hesitated, resmbling dear old dad or not.
In Act II, scene 2, after she has returned the daggers and smeared the grooms with blood, she tells Macbeth, "My hands are of your color, but I shame/To wear a heart so white." This would again not be the statement of an evil person. She discovers that words are one thing but the actual act is quite another.
I would also contend that she is much younger than her husband. Macbeth realizes that if he kills the king, the action will eventually boomerang back at him. Lady Macbeth on the other hand naively states, "A little water clears us of this deed." If she were truly evil, her guilt would not drive her insane and eventually to suicide.
She feels guilt not just for Duncan's murder but also for Banquo's and the Macduff family. Although she wasn't ignorant of the subsequent deaths, she feels guilty since she got the ball rolling by pushing her husband to kill Duncan.
Does Lady Macbeth make some terrible decisions that lead to "evil"? Yes. Is Lady Macbeth evil? No, I would contend that she is a human being who loves her husband very much. She believes in him and will do anything to help him become king. She believes that once he is king life will be wonderful. This, of course, is not realistic.
Remember, Shakespeare does not write black or white characters. His characters are recognisable human and they make mistakes but I can't think of a single character in Shakespeare that is all black or all white, they are all shades of gray, and very human.