One fell swoop Introduction
I'm Macduff. I'm a loyal Scottish nobleman, and I'm willing to fight for my country—no matter what. When I see someone abuse the crown, I'm going to do something about it. And you know what I think?
He has no children. All my pretty ones?
Did you say all? O hell-kite! All?
What, all my pretty chickens and their dam
At one fell swoop? (4.3.215-218)
Who Said It and Where
When news first breaks that King Duncan has been murdered, panic ensues. But not for Macduff. He's the only one who asks why Macbeth killed the guards senselessly. He's also the first to see that Lady Macbeth is fainting. He suspects that something is up.
It turns out that he's right. Macbeth has murdered King Duncan to take the crown from himself. Only instead of prattling on about his suspicions (we're looking at you, Hamlet), Macduff decides to leave for England. This isn't a cowardly act, but rather a brave one intended to aid Malcolm in enlisting the English against Macbeth.
So Macduff and friends are hanging out in England, deciding what to do about Macbeth. The new king's gone on a power trip and is killing everyone who threatens the precious crown on his head. (And even some people who aren't threatening it, like Macduff's family, but he doesn't know that yet. Shhh.)
Ross shows up and chats with Malcolm and Macduff about how Scotland is in a bad way. Macduff asks after his family, and Ross lies that they're fine. He adds that if Macduff were to return, Scotland might gather and take up arms against Macbeth.
Malcolm promises when they finally arrive in Scotland, ten thousand English soldiers will come, too. Then Ross announces he has some bad news, actually. Macduff offers to guess at it, but before he does, Ross blurts out that, oops, actually Macduff's family has been gruesomely murdered.
Macduff blames himself for leaving, but Malcolm recommends that Macduff take his own advice and get his feelings out by murdering rather than weeping. Macduff vows to slay Macbeth, committing to action instead of thought.