It should be noted in this regard that Shakespeare intentionally inserts a scene (Act III, Sc. ii) after Macbeth finishes his conversation with the murderers. In this scene the audience expects discussion between Macbeth and Lady Macbeth about Macbeth’s plan to kill Banquo. But Macbeth intentionally does not speak about his plan. On the contrary he asks Lady Macbeth to specially look after Banquo in the banquet. Shakespeare has intentionally done this because he has wanted to project Macbeth as a Machiavellian king. Macbeth has received foul advices from Lady Macbeth before he has received his kingship. Now the evil Macbeth as a king will act alone – Lady Macbeth has already killed the ‘fair’ residing in him.
Macbeth says to Lady Macbeth, "Be innocent of the knowledge, dearest chuck, / Till thou applaud the deed." He is telling her that he doesn't want her to know until after the deed and then she'll be pleased with what he's done. There are a couple possible reasons for why he isn't telling her. It could be that he wants her to be proud of him for having the ability to plan and carry out a murder on his own. In Act 1, sc. 7, when Macbeth suggests to his wife that they not go through with the murder of Duncan, she essentially calls him a wimp. She attacks his masculinity and her tactic worked because Macbeth decides to go along with her. She is the one who devised the plan, too. So it is entirely possible that he is trying to impress her. Another possible reason for not telling Lady Macbeth of his plan to have Banquo and Fleance killed is "plausible deniability", but I have more belief in the first reason.
Macbeth has further entered into an evil mindset, more capable of committing a crime on his own, and much more willing, so as to not be disgraced by his wife. In the beginning, Lady Macbeth was the one to control Macbeth (more or less) and in a reversal of character qualities portrayed by Shakespeare, Macbeth becomes the onw witht the greater evil and the one witht the greater motive - to fulfill his 'fortune' as king.