What is the evidence that Lady Macbeth does not know that Macbeth will kill Banquo?
In Act 3, Scene 2, Macbeth and his wife discuss their mutual problem involving Banquo. They have attained their objective of becoming king and queen, but they dread the thought that Banquo will profit by their crime in having his descendants rather than theirs become the rulers of Scotland. Macbeth assures his wife in cryptic terms that something will be done about that.
Ere the bat hath flown
His cloister'd flight; ere to black Hecate's summons
The shard-borne beetle with his drowsy hums
Hath rung night's yawning peal, there shall be done
A deed of dreadful note.
When she asks him specifically, "What's to be done?" he tells her:
Be innocent of the knowledge, dearest chuck,
Till thou applaud the deed.
This is the best evidence in the text that Lady Macbeth does not know that her husband will kill Banquo. Macbeth is not concerned about keeping his wife in ignorance of his plans in order to protect her from being an accessory or in order to insure that she does not inadvertently arouse suspicions. Shakespeare has already made his plans perfectly clear in Act 3, Scene 1, and the playwright does not want to risk boring his audience with having everything spelled out to Lady Macbeth which the audience already knows. The words "dearest chuck" may have been intended to suggest that Macbeth had suddenly become protective of his sweet, sensitive wife, but the term of endearment is a weak excuse Shakespeare employs to get out of having to have Macbeth explain what he has already arranged to have done and what the audience will actually see being done in the very next scene when Banquo and his son Fleance are ambushed by the three murderers.
So at this point in the play, Lady Macbeth does not know that her husband intends to have Banquo (and Fleance) killed. Shakespeare probably intended to establish this fact in this scene because she will be all the more bewildered when, in Act 3, Scene 4, her husband sees Banquo's ghost occupying his place at the coronation banquet. She still does not even know Banquo is dead.