In Macbeth, do you find Macduff's behavior in reaction to the news of his family's murder realistic?

This is an interesting question.  It would be simple to
imagine that one can evaluate the behaviour of a character in a play based upon simply
reading the script, but the behaviour of a character in a play is not a complete event
until an actor takes up the part and performs in on stage.  This is because the actor's
choices -- how loud or softly he speaks his lines, whether he is moved to tears or not,
where he stands or sits on stage and in what proximity to the other characters in the
scene, etc. -- all of the actor's choices in combination with the lines that they speak
create the behaviour of the character.

One can evaluate how
"realistic" the lines that Shakespeare has written seem to be, but this shouldn't be
confused with evaluating the character's behaviour.  This is one of the reasons why
reading a play can be a tricky thing to do.  It is tempting to imagine that the text
itself is 100% of the play, but scripts aren't meant to provide the action of the
characters.  The action, or behaviour, is chosen by the actors performing the play.  And
this element, omitted when one simply reads the text, is crucial to a complete
understanding of any play.

Let's look at the text that
comes in response to the news of Macduff's family's murder.  When, in Act IV, scene iii,
Ross begins to tell Macduff of the murders, Malcolm speaks to
Macduff:

readability="9">

Merciful
heaven!

What man!  Ne'er pull your hat upon your
brows.

Give sorrow words.  The greif that does not
speak

Whispers the o'er-fraught heart and bids it
break.

These lines are
significant, even though they are not spoken by Macduff.  Shakespeare has done something
here he often does.  He's given lines to one character that suggest the behaviour of
another.  Malcolm cautions Macduff not to hide silently beneath his hat, but to speak
out his feelings.

Macduff, at first, has short, seemingly
unemotional responses:

My
children
too?

and

And
I must from thence.

My wife kill'd
too?

But his emotions seem to
grow as the realization really takes hold:

readability="9">

. . .All my pretty
ones?

Did you say all?  O, hell-kite! 
All?

What, all my pretty chickens and their
dam

At one feel
swoop?

It is hard to know the
behaviour of Macduff during these lines,  but the text indicates that he is having
trouble taking the news in, since he repeats the information as
questions.

All of this seems very realistic to me -- to
first be in such a state of shock that one cannot say a thing, and then to only be able
to question the information.  Macduff speaks as a man who has received overwhelming news
and simply cannot take it in.  He says to Malcolm that he must "feel it as a man," which
suggests that he cannot consider revenge until he has really taken in the enormity of
his loss.  Shakespeare has created a very realistic sequence of text for Macduff, I
think, a sequence that displays the journey it must be to really take in being
blindsided by the news that one's entire family has been
murdered.