Now comes a period in the drama in which deed follows so fast upon deed,
that there is scarcely any opportunity for characterization. To the
casual view Macbeth seems almost to change his nature, passing from
murder to murder quickly if not easily. He not only arranges for
Banquo's assassination, but leaves Lady Macbeth innocent of the
knowledge. The explanation of this seeming change of character is at
hand. Shakespeare took the history of Macbeth from Holinshed's
Chronicle, and there it is recorded that Macbeth murdered Banquo and
many others, as well as Macduff's wife and children. Holinshed makes
Duncan have "too much of clemencie," and Macbeth "too much of crueltie."
Macbeth's actions correspond with his nature in Holinshed; but
Shakespeare first made Macbeth in his own image--gentle, bookish and
irresolute--and then found himself fettered by the historical fact that
Macbeth murdered Banquo and the rest. He was therefore forced to explain
in some way or other why his Macbeth strode from crime to crime. It must
be noted as most characteristic of gentle Shakespeare that even when
confronted with this difficulty he did not think of lending Macbeth any
tinge of cruelty, harshness, or ambition. His Macbeth commits murder for
the same reason that the timorous deer fights--out of fear.

"To be thus is nothing;
But to be safely thus. Our fears in Banquo
Stick deep, and in his royalty of nature
Reigns that which would be feared":

And again:

"There is none but he
Whose being I do fear":...