What was Big Willy Shakes going for?
Macduff isn't a man of many words, which means everyone really listens when he talks. He's just found out that his family is gone. Malcolm tells him to man up. Fight Macbeth instead of crying about it like a little girl. But instead of pulling a Macbeth and being self-conscious about his masculinity, Macduff says sure, he's man up. But he's also going to "feel it as a man."
Basically, in Macduff we see a guy who can feel and act. He's not afraid to show emotion at the right time, and this scene is one of those times. The poor guy is understandably devastated that his family is dead. But don't go thinking he's a tearful sissy. He still brings down Macbeth in the end.
His phrase "one fell swoop" is referring to his entire family. He's asking: not all of them, right? Surely Macbeth didn't have my little kids killed, too? Spoiler alert: he did. Macduff's frustration, anger, and sheer horror at the needless cruelty of his children dying come out in his mini speech in this scene.
His word choice points at this, too. "Swoop" is what a bird does when it's attacking its prey. It dives in abruptly and without delay. The word "fell" is from an old English word meaning savage or ruthless. It's the same word that we get felon from today.
So when Shakespeare created this line for Macduff, we're guessing he had these in mind. If you think about it, "one fell swoop" really means that Macbeth quickly killed all of Macduff's family in a terrible way.
And we totally feel for the guy. He just lost his entire family.