When Macbeth heard that Banquo’s children would inherit
the throne, Macbeth was not pleased. Macbeth did not want Banquo’s sons having the throne because he wanted his kingship
to be passed down to his own sons. So, Macbeth makes a plan to kill Banquo with
some hired assassins:
Murderers: We
are resolved, my lord.
Macbeth: ... It is
concluded. Banquo, thy soul’s flight,
If it find heaven, must find it out tonight (Shakespeare III.2.159-162).
Thus, Macbeth goes through the extreme measure of killing Banquo, by ordering the assassins
to ensure that Banquo is killed that same night.