Why is Macbeth so mad that Fleance wasn’t killed too?

Fleance is one of the sons of Banquo. Because in act 1 the three witches tell Macbeth that Banquo "shalt get kings" (be the father of kings), Macbeth is necessarily concerned with the possibility that Banquo's sons, rather than Banquo himself, will usurp his power. As such, he arranges to have Banquo and Fleance killed, stating that "Fleance's absence is no less material to me / Than is his father's" (that is, it is just as important to me that you kill Fleance as that you kill Banquo). Arguably, it is actually even more important that Fleance himself be disposed of, because the witches have already alerted Macbeth to the fact that it is Fleance, rather than Banquo, who will occupy the throne.
Lennox, when explaining how "Fleance fled" in act 3, scene 6, suggests that they could simply say that Fleance had murdered his father, making it thus convenient enough for them that he has gone, but Lennox fails to grasp the complexity of the situation for Macbeth. As far as Macbeth is concerned, it is of vital importance that Fleance be removed from the situation, because while Fleance lives, Macbeth has to worry about him returning to take Macbeth's throne as was prophesied (especially because Macbeth has come to believe in the veracity of the prophecy by this point in time). In the historical context, the Elizabethan audience would have recognized the character of Fleance as the legendary ancestor of the House of Stuart, who did become kings of Scotland (and, later, England, after Elizabeth I's death).