Lady Macbeth is not pleased with this turn of heart, which she considers cowardly on Macbeth's part. Just...
In act 3, scene 7 of Shakespeare's Macbeth, Macbeth is having second thoughts about murdering Duncan. He tells Lady Macbeth, "We will proceed no further in this business" (3.7.34).
Lady Macbeth is not pleased with this turn of heart, which she considers cowardly on Macbeth's part. Just a few minutes ago, Macbeth seemed ready, willing, and able to murder Duncan, and now he's backing out of the deal. Macbeth's excuses don't help to make him appear any less cowardly in Lady Macbeth's eyes.
MACBETH. ...He hath honor'd me of late, and I have bought
Golden opinions from all sorts of people,
Which would be worn now in their newest gloss,
Not cast aside so soon. (1.7.35-38)
Lady Macbeth isn't impressed with Macbeth's feeble excuses, and goes straight to the heart of the matter. Clearly, Macbeth doesn't love her, she says. She goads him, saying that he's afraid to put his desires into action and prefers to live the life of a coward.
LADY MACBETH. Wouldst thou have that [the shiny "golden opinions" from all sorts of people"]
Which thou esteem'st the ornament of life
And live a coward in thine own esteem,
Letting “I dare not” wait upon “I would”
Like the poor cat i’ the adage? (1.7.45-49)
Is Macbeth a "'fraidy cat"? Macbeth meekly protests Lady Macbeth's labelling of him as a coward.
MACBETH. I dare do all that may become a man;
Who dares do more is none. (1.7.51-52)
Wrong answer. Lady Macbeth will have none of this whiny attitude from Macbeth, and she attacks his manhood with a vengeance. Macbeth is helpless in the face of this onslaught, but he makes one more feeble attempt to justify his reluctance to kill Duncan.
MACBETH. If we should fail? (1.7.66)
Interestingly, Lady Macbeth's response to this question is given alternately as "We fail?" (in the First Folio of 1623) and "We fail!" (in some subsequent editions of the play). If "We fail?" Lady Macbeth is firmly rejecting any notion that they'll fail. If "We fail!" Lady Macbeth is firmly rejecting any consequence of failing, much preferring to try and fail, no matter the outcome, rather than never to try at all.
Either way, Macbeth realizes that he's totally out of his league trying to argue manhood with Lady Macbeth, and he succumbs to her will, as she no doubt expected he would.
In act 3, scene 1, the newly crowned Macbeth worries to himself about Banquo and about Banquo's descendants, who the witches told Banquo would be kings.
A servant enters with two murderers with whom Macbeth has apparently already met, and to whom he's told lies to about Banquo being the reason for their lowly station in life.
Although it seems in this scene as though Macbeth is trying to convince these two men to become Banquo's murderers, they're already identified in the play as "two Murderers" (or "two Murtherers" in the First Folio), rather than "two men from the neighborhood."
These two men are already in the business of murdering people, as their identification as "two Murderers" (or "Murtherers") strongly implies. They're likely predisposed to do what Macbeth asks them to do, since they're eager to reap any reward that might come their way for doing the new king's bidding. They're probably hoping for repeat business and glowing referrals to other would-be tyrants.
Nevertheless, for some reason, Macbeth goes through the motions of convincing them to kill Banquo. The First Murderer seems to resent Macbeth's insinuations that they're too meek and too religious to do anything about Banquo's mistreatment of them.
FIRST MURDERER. We are men, my liege. (3.1.97)
Macbeth learned his lesson about manhood from Lady Macbeth, and here he tries to put that lesson to use against the murderers. After all, the murderer brought up the business about being men first, like Macbeth first brought it up to Lady Macbeth, so it's Macbeth's chance to turn the manhood tables on the murderers.
MACBETH. Ay, in the catalogue ye go for men,
As hounds and greyhounds, mongrels, spaniels, curs,
Shoughs, waterrugs, and demi-wolves are clept
All by the name of dogs. ... and so of men.
Now if you have a station in the file,
Not i’ the worst rank of manhood, say it,
And I will put that business in your bosoms (3.1.99-111)
"You look like men," says Macbeth, "but there are all kinds of men, just like there are all kinds of dogs. Prove to me that you're the right kind of men, and I'll let you perform this murder for me."
The murderers aren't so easily cowed by Macbeth as Macbeth was cowed by Lady Macbeth. They each state their case without trying to justify themselves to Macbeth.
SECOND MURDERER. I am one, my liege,
Whom the vile blows and buffets of the world
Have so incensed that I am reckless what
I do to spite the world.
FIRST MURDERER. And I another
So weary with disasters, tugg'd with fortune,
That I would set my life on any chance,
To mend it or be rid on ’t. (3.1.116-123)
Macbeth finds them apt, and they get the job.
SECOND MURDERER. We shall, my lord,
Perform what you command us. (3.1.138-139)
This was what the two murderers intended from the beginning, of course, but Macbeth apparently felt a need to test them for reasons known only to himself. The question arises, however, that if Macbeth decided that these were the men for the job, why then did he send another murderer to join them in murdering Banquo?
In act 3, scene 3, the two murderers arrive at the appointed time and place with a third murderer.
FIRST MURDERER. But who did bid thee join with us?
THIRD MURDERER. Macbeth. (3.3.1-2)
No further explanation is requested or given as to why the third murderer appears to assist the other two murderers. It might be that even after vetting the two murderers, Macbeth still didn't trust them to carry out the murder.
Perhaps Macbeth knows, being a murderer himself, how untrustworthy murderers are. He understands from personal experience how they might first agree to do a murder, only to have second thoughts and try to back out of the job. Macbeth may have simply wanted to avoid that possibility.