I don't think that Macbeth actually goes mad at any point in the play. He certainly becomes more violent, blood-thirsty, and tyrannical, but there's no sense that he's lost his mind; he always knows exactly what he's doing and why, even if there seems to be no rhyme or reason to his actions.
So in order to answer this question we need to look for relevant quotations that illustrate Macbeth's descent, if not into madness, then into self-delusion brought on by his increased feeling of invincibility.
One such example comes in act 4, scene i, just after Macbeth's clapped eyes on the apparition of what looks like a bloody child. The apparition tells Macbeth that no man born of a woman will ever harm Macbeth. Macbeth responds by downplaying the threat of Macduff:
Then live, Macduff. What need I fear of thee?
But yet I’ll make assurance double sure,
And take a bond of fate. Thou shalt not live,
That I may tell pale-hearted fear it lies,
And sleep in spite of thunder.
Macbeth is so obsessed with the...
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