This is an immensely revealing scene that shows the spiral of corruption and evil that Macbeth has entered into and how he is descending rapidly downwards towards ever greater moral degradation. What is key to note about this scene is the way in which Macbeth and Lady Macbeth have, in some ways, exchanged places. Now it is Macbeth who is plotting to kill Banquo and Fleance by himself, whereas before he needed her assistance and encouragement. He even goes as far as to not involve her in any way, telling her, politely, to mind her own business:

Be innocent of the knowledge, dearest chuck,

Till thou applaud the deed.

This shows the extent to which Macbeth has embraced evil. In addition, just after saying this, Macbeth makes an imprecatory speech to night, in much the same way that Lady Macbeth abandoned herself to the forces of evil in Act I scene 5. Note his words:

Come, seeling Night,

Scarf up the tender eye of pitiful Day,

And, with thy bloody and invisible hand,

Cancel and tear to pieces that great bond

Which keeps me pale!

This clearly represents an attempt by Macbeth to fill his whole body with darkness and evil. We see a greatly changed Macbeth. He now needs no Lady Macbeth to cajole and shame him into action. He has embraced evil, and shows clearly how one bad act leads to ever greater transgression.