Final answer:
In Shakespeare's 'Macbeth', Macbeth murders Banquo, not because Banquo was next in line to be King after Duncan, but because of the witches' prophecy that Banquo's descendants would be kings, and thus, a threat to Macbeth's power.
Step-by-step explanation:
In Shakespeare's Macbeth, the Scottish thane murdered by Macbeth because he feared his offspring would be kings, and thus potentially a threat to Macbeth's claim to the throne, is Banquo.
Banquo is Macbeth's co-general in the wars and his close companion at the beginning of the play. After Macbeth becomes the Thane of Cawdor, thanks to the prophecies of the witches and the subsequent events that lead to King Duncan's death, Macbeth feels threatened by the prophecy that Banquo's descendants will become kings. Consumed by his paranoia and desire to secure the throne, Macbeth arranges the murder of Banquo and attempts to kill Banquo's son, Fleance, who ultimately escapes.
It is noteworthy to mention that while Banquo was never in line to be King after Duncan, Macbeth's fear comes from the prophecies predicting that Banquo's lineage would take the throne, which leads Macbeth to commit the heinous act of betrayal against Banquo.