In Macbeth Act 2 scene 3, what excuse does Macbeth make for killing the guards, and how does he make it?

This is actually a very interesting point in the play.
Having gone in to see the body of Duncan, and seeing the grooms covered in blood, thanks
to Lady Macbeth, Macbeth kills them immediately, before they have had a moment to talk
or testify. Of course, this immediately covers up his involvement in the death of Duncan
and means that the guilt of the grooms cannot be disproved. However, note what Macduff
says very suspiciously after Macbeth reveals that he killed
them:

Wherefore did you
so?

Depending on how you
stage such a comment this could seriously unsettle Macbeth so much that he stammers out
the rather pathetic excuse that follows:

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Here lay
Duncan,

His silver skin lac'd with his golden
blood;

And his gash'd stabs look'd like a breach in
nature

For ruin's wasteful entrance: there, the
murtherers.

Steep'd in the colours of their trade, their
daggers

Unmannerly breech'd with
gore.

The imagery is
overblown and exaggerated, giving his speech an insincere tone and emphasising the fact
that he is desperately trying to come up with some reasonable explanation for his highly
suspicious act. Hyperbole is used extensively, and the weakness of this speech is
demonstrated by Lady Macbeth, who recognises what a bad job her husband is doing at
giving an excuse and faints as a result, creating a diversion to distract everyone from
his words.