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:
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.