- In Act III, Scene 1, Macbeth lets the murderers think that Fleance's murder is an afterthought, just to make things clean and tidy.
Do you think the First and Second Murderers would have acted differently in Act III, scene 3 if Macbeth had told them how crucial Fleance's death is to him?
Because Macbeth has said that Fleance's death is secondary to him, the murderers, common men who are down on their luck with families to support, would probably have thought nothing about the boy's escape. Because Macbeth has convinced them that the dire circumstances in which they now find themselves are Banquo's fault (when they are really Macbeth's), this other knowledge might well have motivated them to kill Fleance as well.
Have you consider'd of my speeches? Know
That it was [Banquo], in the times past, which held you
So under fortune, which you thought had been
Our innocent self? This I made good to you
In our last conference, pass'd in probation with you(85)
How you were borne in hand, how cross'd, the instruments,
Who wrought with them, and all things else that might
To half a soul and to a notion crazed
Say, “Thus did Banquo.” (III.i.81-89)
I find this especially true in that their children are starving. They have no love for Banquo, and I think that poverty and Macbeth's manipulations have so blinded them, that they might well allow their frustrations to end Fleance's life, had they known his real importance: for it would have been one more way to make Banquo, their "enemy," pay.
There are three murderers and two victims; it is late evening since "The west yet glimmers with some streaks of day" and they need a "torch" ("Enter BANQUO, and FLEANCE with a torch"). So what are possible actions the murderers took that may have left room for different behavior in III.iii had they had different information, although Macbeth does give significant weight to Fleance's death when he speaks of it in III.ii:
Since it was 3-to-2, the murders might have split up the work with one attending to Fleance while two took Banquo. Had they had more weighted impressions of Fleance's death, this is one difference that might have occurred. Additionally, if light was important to their task--and they could not have brought their own without giving themselves away--one should have had the responsibility of securing the light before it went out "Who did strike out the light?" / "Wast not the way?"
Of course, it is hard to surmise what characters in dramas would think when there has been little or no development of these personages. However, killing a child is usually not something that even blackguards are eager to do. While Macbeth has convinced them that Banquo has held them "So under fortune," they might not feel enough resentment toward Banquo's son.
The murderers are certainly not the skillful mercenaries we see in the movies today; they are simply a surly, sour bunch of lowlifes. They are the best that a desperate could procur on such short notice without arousing suspicion. Whether or not they would have succeeded in killing Fleance is hard to say, but surely the murderers would have been more afraid to return to Macbeth and admit that Fleance had escaped if Macbeth had been more emphatic that he should die as well.
As it is, the murderers think that Banquo is their enemy, that he is the source of their poverty and bad luck. Macbeth cannot rationally extend that reasoning to include Fleance, a young child. Unfortunately, Macbeth cannot admit that he has been visiting witches that promise Banquo to be the father to a line of kings either. All in all, Fleance is part of a prophecy, so his endurance is foreshadowed from the beginning.