How does Macduff react in Act 4 Scene 3 when he hears that his family has been slaughtered?

Ross is evidently concerned that, when he passes on his message to Macduff, Macduff will blame the messenger—"despise my tongue for ever"—and perhaps react with anger to the tragic news. However, instead, what we see from Macduff is disbelief. He appears to be in a state of shock. After Ross has already told him that his "wife and babes" were "savagely slaughter'd," Macduff repeats, "My children too?" as if he cannot credit the news. Next, he asks after his wife—"My wife kill'd too?"—although Ross has already confirmed that this is the case. The news is simply too overwhelming for Macduff to comprehend, and he returns continually to questioning the fact of the matter:

O hell-kite! All?What, all my pretty chickens and their damAt one fell swoop?

Malcolm tells Macduff to "dispute it," or bear it, "like a man," but Macduff counters that he will do so, "but I must also feel it as a man." Malcolm here means something different by "man" to what Macduff means. He is telling Macduff to be strong, whereas Macduff is saying that, as a feeling human person, he has emotions which he needs to process. Ultimately, however, he does take Malcolm's advice to turn his grief into action, declaring

Bring thou this fiend of Scotland and myself;Within my sword's length set him; if he 'scape,Heaven forgive him too!

Macduff is obviously devastated by the news that his family has been slaughtered, and he is portrayed as a man in shock, a loving husband and father who is understandably wrecked by this blow. He rejects the idea that he should appear unfeeling in order to be a "man." Instead, he utilizes his grief to galvanize him in his continued battle against Macbeth.

In act 4, scene 3 of Macbeth, Macduff is hiding out in England, having fled Macbeth's incipient tyranny in Scotland. Ross enters and tells Macduff the terrible news: his whole family has been murdered on the orders of Macbeth. As we would imagine, Macduff is absolutely devastated by the news. But Malcolm urges him to man up and turn his grief to righteous anger:

Be comforted.
Let’s make us med'cines of our great revenge,
To cure this deadly grief.

And also says:

Dispute it like a man.

Macduff will do so; he will act according to the conventions of how an aristocratic male should behave. But at the same time, he won't hide his human feelings; he cannot forget the precious memories of his family. His ensuing revenge against Macbeth won't just be political; it will be personal too.
 
Macduff's reaction is also tinged with guilt for having abandoned his family, leaving them all alone and vulnerable to Macbeth's terrible vengeance:


Sinful Macduff,
They were all struck for thee! Naught that I am,
Not for their own demerits, but for mine,
Fell slaughter on their souls. Heaven rest them now.



So when Macduff finally confronts Macbeth and kills him in a duel, he isn't simply carrying out an act of revenge; he is also going some way to assuage the terrible guilt he still feels over the way in which he abandoned his family to their terrible fate.