Once the crown is set on Mackers head, in blazing theatre speed, after Duncan's murder, the question nearly immedietaly becomes how to keep it there, as he himself indicates, with safety. There is no interim in which to enjoy the royalty presented to him, no time for peace and appeasement and any kind of reconciliation, before he must weild his grave authority and become that which he may have perhaps sought to avoid with assumption of that golden round.
In some rehearsal or other I became aware of the hypocrisy of the crown and viscrelly felt a disgust to the clothes and jewelry I was wearing. It seemed that there were daggers in the smiles of my courtiers, that they might actually be seeing through this duplicity, and nearly every word of mine becomes loaded with qualification and inquest.
Banquo's eradication seems the most obvious and clear path to a new equilibrium with the additional weight of the crown upon his head. Of course, Banquo's murder is the fourth murder of the play, or at least the fourth that is known to the audience, so there is a bit of irony underlying the statement "There is none but he, whose being I do fear..." Its almost a laughable comment, and perhaps in the next run or so, I could even second guess the statement by giving the audience a suspicious look. Indeed it is NOT just he whose being I do fear. Or at least his being I do fear only for the present, who knows what tomorrow may bring. Well, my point is that this seeming final and last step towards stability is futile, and perhaps even self-conciously indefinite.
This paradigm is demonstrated in miniature in the behaviour of the two men who are brought in to murder Banquo. Under seeming worthy pretense, and holy motivation, they are coerced into the plan. Very much perhaps like Mac was coerced into his seminal murder. The plan itself is of little importance as much as their involvement. But in such acts, one can only go in for a mile, and before these gentlemen have a second to truly resolve themselves they are bound to another murder, of an unquestionably innocent creature, Fleance. Similarly, Mac's intitial murder leads to still other murder. If there was any chance of these men bearing any honorable ilk to their grave at the start of the scene, that ilk is but blown athwart by the end of the scene. If there was any chance that these characters were concerned citizenry at the top of the scene, by the bottom they are murderers in character. The corruption ranging in Mac is spread outward and downward through the heirarchy of his government like a plague. Tyranny does not lie alone in the tyrant.