Final answer:
Macbeth is alarmed by the witches' prophecy because it predicts that Banquo's heirs, not his own, will inherit the throne, making his crimes pointless and threatening his power.
Step-by-step explanation:
The lineage of kings that the witches show Macbeth alarms him because it signifies that his own descendants will not inherit the throne. Instead, the witches' visions suggest that the heirs of Banquo, Macbeth's friend and fellow warrior, will become rulers.
This is particularly troubling for Macbeth, as he has committed regicide to become king, and the prospect of his lineage not continuing his royal legacy means that his efforts and crimes were in vain.
Additionally, Banquo's heirs ascending the throne threatens Macbeth's power and security, heightening his paranoia and leading to a further spiral into tyranny.