What reason does Macbeth give for not going and killing Banquo himself?Macbeth hands the dirty work to the murderers, but what is it that stopped...

In Act III Scene 1 of Macbeth by Shakespeare, Macbeth is
told by Banquo that he has all the titles now as the witches have predicted, but he also
says,

...and
I fear

Thou play'dst most foully for
't. 

Yet it was said'It should not stand in thy
posterity,

But that myself should be the root and
father

Of many kings....
(3.1.2-5)

When Banquo says
that he expects to have the propheses of the "weird women" come true for him, Macbeth in
fear determines that

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To be thus [King]is nothing, but [unless] safely
thus

Our fears in Banquo stick deep.
(3.1.47-48)

So, in order to
ensure that he will remain king, Macbeth orders the murders of Banquo and his son
Fleance.  He tells the murderers that he could perform the act except for the fact that
he and Banquo have mutual friends, friends who Macbeth must court for they would protest
the death of Banquo.  So, he must hide this act from
them:

...and
though I could

With barefaced power sweep him from my
face 

And bid my will avouch it, yet I must
not

For certain friends are both his and
mine,

Whose loves I may not drop, but wail his
fall

Who I myself struck down. And thence it
is

That I to your assistance do make
love,

Masking the business from the common
eye

As he talks with the
hired murderers, he tells them where to put themselves and expresses the urgency of
their killing Banquo and his son.  Also, Macbeth instructs them to be sure to perform
their deeds away from the palace, remembering that he

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...require(s) a clearness; and with
him--

To leave no rubs nor botches in the
work--(3.1.133-134)

Macbeth
wants no suspicion of his being involved in the murder of Banquo and Fleance because he
wishes to remain as King.