Stepped in so far, that, should I wade no more,
Returning were as tedious as go o'er."
[Footnote 1: It seems to me probable that Shakespeare, unable to find an
adequate motive for murder, borrowed this one from "Richard III." Richard
says:

"But I am in
So far in blood that sin will pluck on sin"--

This is an explanation following the fact rather than a cause producing
it--an explanation, moreover, which may be true in the case of a
fiendlike Richard, but is not true of a Macbeth.]

Take it all in all, this latter reason is as poor a motive for
cold-blooded murder as was ever given, and Shakespeare again feels this,
for he brings in the witches once more to predict safety to Macbeth and
adjure him to be "bloody, bold and resolute." When they have thus
screwed his courage to the sticking place as his wife did before,
Macbeth resolves on Macduff's murder, but he immediately recurs to the
old explanation; he does not do it for his "own good" nor because
"returning is tedious "; he does it

"That I may tell pale-hearted fear it lies,
And sleep in spite of thunder."

It is fair to say that Shakespeare's Macbeth is so gentle-kind, that he
can find no motive in himself for murder, save fear. The words
Shakespeare puts into Hubert's mouth in "King John" are really his own
confession: