In act 4, scene 1, Macbeth hears that Macduff has fled and is sorry he didn't kill him when he had the chance. He states that from now on, the very moment he has a thought, he will act on it. In fact, he says, he will immediately have Macduff's...
In act 4, scene 1, Macbeth hears that Macduff has fled and is sorry he didn't kill him when he had the chance. He states that from now on, the very moment he has a thought, he will act on it. In fact, he says, he will immediately have Macduff's wife and children killed—which he does.
This is a contrast to the thought he expends on killing Duncan. He agonizes over that decision and rightly anticipates that he will be forever living in bloodshed once he starts down that bloody path. He even second-guesses himself and decides not to act, until his wife goads him into it.
He acts faster with Banquo, but not without forethought. He has the motive of Banquo knowing too much (he heard the witches' prophecies too), and he rightfully suspects that Banquo fears he is behind Duncan's death. He is also angry that he himself has taken on all the risk and guilt—and already is unhappy wearing the crown that he thought would make his dreams come true—and yet Banquo's children, he believes, will reap the rewards. He has good reason to kill Banquo, from his point of view.
The killing of a helpless woman and her children is a new low and an act of impulse, as Macbeth himself states when he says the "firstlings" (first thoughts) of his heart with be the first acts of his hand:
The very firstlings of my heart shall be
The firstlings of my hand. And even now,
To crown my thoughts with acts, be it thought and done:
The castle of Macduff I will surprise,
Seize upon Fife, give to th' edge o' th' sword
His wife, his babes, and all unfortunate souls
That trace him in his line.
This is a descent into tyranny, a picture of what tyrants do: they act on the impulses of the moment without sober contemplation. Throughout his oeuvre, Shakespeare shows he has far more respect for leaders who think before they act. Macbeth, in contrast, shows he is totally hardened with this declaration privileging impulse.