Never for a moment, even in the very article of the murder, does he possess his own soul.

He is a man on wires.

From first to last it is an exhibition of hideous cowardice.

For, after all, it is not here, but in broad daylight, with the exhilaration of conflict, where he can assure himself at every blow he has the longest sword and the heaviest hand, that this man's physical bravery can keep him up; he is an unwieldy ship, and needs plenty of way on before he will steer.
In the banquet scene, while the first murderer gives account of what he has done, there comes a flash of truculent joy at the 'twenty trenched gashes' on Banquo's head.

Thus Macbeth makes welcome to his imagination those very details of physical horror which are so soon to turn sour in him.