What excuse or explanation does Macbeth give for killing the guards (grooms)? What is his real reason?
In Shakespeare's Macbeth, the protagonist Macbeth makes a strategical error when he kills Duncan's grooms after Duncan's body is discovered. In short, he varies from his wife's plan and, thus, raises doubts as to his innocence and guilt in at least one other thane's mind, Macduff's.
When Macduff asks Macbeth why he killed the grooms (likely the only witnesses to Duncan's murder), Macbeth attempts to explain away or rationalize his actions:
Who can be wise, amazed, temp'rate and furious,
Loyal and neutral in a moment? No man.
Macbeth excuses his actions here by asking a rhetorical question that emphasizes the contrasting emotions he felt when he saw Duncan's bloody body. No man can feel these emotions at the same time and act reasonably, suggests Macbeth.
He then elaborates on the same idea:
Th' expedition of my violent love
Outrun the pauser, reason.
His violent love made him act without pausing to think.
...Here lay Duncan,
His silver skin laced with his golden blood,
And his gashed stabs looked like a breach in nature
For ruin's wasteful entrance; there the murderers,
Steeped in the colors of their trade, their daggers
Unmannerly breeched with gore. Who could refrain
That had a heart to love, and in that heart
Courage to make's love known?
Macbeth saw the bloody Duncan, he explains, and he saw the grooms steeped in the blood they caused to flow, and he could not resist killing them. No one that loved Duncan as Macbeth did could stop himself from immediately killing his murderers, says Macbeth. Notice with that last thought that Macbeth is trying to redirect the argument in his favor, establishing that his slaying of the grooms shows his great love for Duncan.
Lady Macbeth swoons soon after. While it is possible she fakes her feinting in order to take the attention away from what Macbeth has just done (killing the grooms), I suggest that she feints because she is in shock due to the blunder Macbeth has just committed.
The witches had presented Macbeth with the prophesy that he would one day be the king. Macbeth's wife desires that it should happen sooner. She manipulates him into going to the king's chambers and killing him so that he can ascend the throne quicker. However, he has to kill him so that others will not think it was him who had murdered the king.
Macbeth kills the guards so that there will have been no witnesses to his murderous actions. He then gains the throne and pretends to be seeking the traitor who had murdered the king.