Well, it can't be that Macbeth is unconcerned with whether Fleance dies or not. In at least two spots (prior to the attempted murder) he mentions how important it is that Fleance die with his father. The first is in a soliloquy in scene one when he considers not having an heir himself, he says:
For Banquo's issue have I filed my mind,
For them the gracious Duncan have I murder'd...
Only for them...
To make them kings, the seed of Banquo kings!
Rather than so, come fate inot the list
And champion me to the utterance!
And then with the murderers, he makes special note of Fleance:
Fleance, his son...
Whose absence is no less material to me
Than his father's, must embrace the fate
Of that dark hour.
Actually, in scene iv, in Macbeth's conversation with the murderer at the door during the banquet, Macbeth doesn't seem indifferent to the news that Fleance escapes at all. When told this he says:
Then comes my fit again. I had else been perfect...
But now I am cabin'd, cribb'd, confined, bound in
To saucy doubts and fears.
...the worm that's fled
Hath nature that in time will venom breed
Which means that he's very aware of the danger the Fleance still alive poses.
So, throughout Act Three, he does see, at least from what he says, that the death of Fleance is quite important and that it's a huge defeat to him when Fleance escapes.
I see no indifference on Macbeth's part to wheter or not he wants Fleance dead. Every time Macbeth mentions Fleance, he sees him as a threat and wants him killed.
Here in act 3, scene one, he speaks to the Banquo. He is planning the murder of both of them and wants to make sure that Fleance will be with his father:
MACBETH:We hear our bloody cousins are bestow'd In England and in Ireland, not confessingTheir cruel parricide, filling their hearers With strange invention. But of that tomorrow,When therewithal we shall have cause of stateCraving us jointly. Hie you to horse; adieu,Till you return at night. Goes Fleance with you?
Here, in the same scene, he tells the murders that it is just as important to kill Fleance as it is for them to kill Banquo:
MACBETH: ...To leave no rubs nor botches in the work— Fleance his son, that keeps him company, Whose absence is no less material to me Than is his father's, must embrace the fate Of that dark hour.
Also in scene 3, Macbeth tells Lady Macbeth of his desire to kill both the father and the son:
MACBETH:O, full of scorpions is my mind, dear wife! Thou know'st that Banquo and his Fleance lives.
LADY MACBETH:But in them nature's copy's not eterne.
MACBETH:There's comfort yet; they are assailable.
And in scene 4, we can see how upset Macbeth is that Fleance has escaped and how he continues to want him dead:
MURDERER:My lord, his throat is cut; that I did for him.
MACBETH:Thou art the best o’ the cut-throats! Yet he's goodThat did the like for Fleance. If thou didst it,Thou art the nonpareil.
MURDERER:Most royal sir,Fleance is 'scaped.
MACBETH:Then comes my fit again: I had else been perfect,Whole as the marble, founded as the rock,As broad and general as the casing air: But now I am cabin'd, cribb'd, confined, bound in To saucy doubts and fears.—But Banquo's safe?
MACBETH:Thanks for that. There the grown serpent lies; the worm that's fled Hath nature that in time will venom breed No teeth for the present.
Oh, no: there is never a doubt as to Macbeth's intentions in regard to Fleance... no room for irony.
Since the prophesy was that Banquo would father a line of kings, in Macbeth's mind he was the greater threat. Fleance is still a boy and many things could happen to him before he would be a threat. As Macbeth says when told of the boy's escape, "...The worm that's fled/Hath nature that in time will venom breed,/No teeth for th' present...'
Macbeth has more important things on his mind, like keeping his ill gotten crown. Fleance can wait until a better time since he is no immediate threat.
The irony is that Banquo does indeed beget a line of kings. Fleance is his only son. No child of Macbeth becomes king. Macbeth realizes in the end that all he has done has benefited Banquo and his heirs not Macbeth.