Macbeth was irrational when he reacted to the witches’ prophecies. He is pretty sure that they will come true once he gets one of them, and is promoted to Thane of Cawdor. At that point, he sort of loses his rationality. When he says that Malcolm is a step he needs to overleap or fall down on, he is being irrational.
Lady Macbeth goes to great lengths to convince her husband to go through with the murder, when he has second thoughts.
"What beast was't, then,
That made you break this enterprise to me?
When you durst do it then you were a man;
And, to be more than what you were, you would
Be so much more the man. Nor time nor place
Did then adhere, and yet you would make both" (Act I, Scene VII)
Macbeth chooses ambition over everything else in life. After he kills Duncan, he loses everything a little at a time. The ability to eat, sleep, the comfort of family and friends.
"Methought I heard a voice cry 'Sleep
Macbeth does murder sleep,' the innocent sleep,
Sleep that knits up the ravell'd sleave of care,
The death of each day's life, sore labour's bath,
Balm of hurt minds, great nature's second
Chief nourisher in life's feast," Act II, Scene II)
"Blood hath been shed ere now, i' the
Ere human statute purg'd the gentle weal;
Ay, and since too, murders have been perform'd
Too terrible for the ear: the times have been,
That, when the brains were out, the man would die,
And there an end; but now they rise again,
With twenty mortal murders on their crowns,
And push us from our stools: this is more
Than such a murder is." (Act III, Scene IV)
After Banquo's murder, Macbeth sees his ghost and starts to lose his mind.
In the end, both Macbeth and Lady Macbeth are swallowed up by fatal consequences to their actions.