Banquo is mad
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Macbeth is angry because Banquo has sons that will be kings and he will not. One of the biggest reasons to be a king was to set your sons to become kings after you die. Macbeth becomes furious that he will have no royal "seed" to carry his name. Having no son really emasculates a king. Macbeth goes on a rant about this in Act 3 sc 1:
They hailed him father to a line of kings.
Upon my head they placed a fruitless crown
And put a barren scepter in my grip,
Thence to be wrenched with an unlineal hand,
No son of mine succeeding.