What is Macbeth talking about in this quote, how does this quote relate to the plot? and what is the significance of this quote?
Who can be wise, amazed, temp'rate, and furious,
Loyal and neutral, in a moment? No man.
Th' expedition of my violent love
Outrun the pauser, reason. 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?
The killing of the grooms is the proverbial beginning of the end for Macbeth. It's important for numerous reasons.
It's the first time Macbeth veers from his wife's plan. From this point on, concerning the plot, Macbeth will make his own decisions, without even consulting his wife. He will shut her out of his decision making. This is a mistake. His wife plans well and sees the big picture, or the consequences of an action. Macbeth does not. When he arranges the murder of Banquo and Fleance, he leaves little doubt that he is guilty of treachery, and he becomes a suspect. His killing the grooms leads immediately to suspicion on the part of Macduff, and Macduff is, of course, the hero of the play who will bring Macbeth down.
Incidentally, while Lady Macbeth's feinting may, indeed, be a stunt to divert attention from her husband's mistake, it may also be an actual incident of feinting: feinting caused by her husband's idiocy at being so stupid as to kill the grooms. She is shocked at his actions. The grooms would have had no information to give except that they were drunk and asleep. That isn't much of an alibi when a king has been killed.
In the quote you ask about Macbeth also provides another example of what appears fair really being foul, thereby furthering that theme. He explains how reason left him and emotion, his love for Duncan, took over. He covers himself by saying that any man who really loved his king could do no other than kill those who killed him. Interestingly, notice how Macbeth indirectly calls himself a hero, here. He not only evades any condemnation for killing the grooms, but makes himself out a loyal Thane and follower of Duncan for having done so. What great Thane wouldn't avenge his king? He thereby claims to love Duncan more than the others--for they controlled their love, while he could not--his love is "violent" love.
Macbeth claims that he couldn't help himself, that reason left him. Duncan's skin is "silver" and his blood "golden." The dagger wounds are like some great breach in nature. The real breach is the unnatural act Macbeth has comitted--which, by the way, contributes to another theme. That of the unnatural.
Macbeth indeed is talking about his murder of the two guards. He claims it was impossible not to lose his temper when he saw Duncan's dead body and the guards with their bloody weapons. He claims that no man would have been able to refrain from killing the guards. Of course, Macbeth is merely pinning the murder of Duncan on the guards. The answer to the question he asks as to whether any man could refrain from killing the guards who seemed to be murderers of Duncan is obvious. Of all the men who saw the murdered Duncan and the drugged guards, only Macbeth felt the need to kill the guards. Macduff even wonders why Macbeth killed the guards.
Macbeth's language here is dramatic and exaggerated. It is full of hyperboles. Lady Macbeth's fainting at the end of this speech is interesting. It could be argued that she faints to divert the attention away from her false-speaking husband in fear he may reveal himself as the murderer through his overstatements.
This quote comes from Act II, Scene 3 of Macbeth. In it, Macbeth is talking about why he killed the two guards who were supposed to be guarding King Duncan's chambers. It is significant because it helps us to see how corrupt Macbeth has become.
Macbeth is saying in this quote that he had to kill the two because he was so angry at them for killing King Duncan. Of course, he knows they did not kill the king because Macbeth was the real murderer. Instead, he is really killing them so that no one can ask them questions. If the two could prove they didn't kill Duncan, questions would be asked and Macbeth might get caught.
So Macbeth kills these two innocent men simply to take away any chance that people will find out he is guilty. That shows how ruthless he is.