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.