that he will not be killed by anyone born of a woman.

Macbeth hears a woman's cry, and Seyton goes to investigate. When Seyton returns, he says simply, “The queen, my lord, is dead.” Macbeth merely remarks that she would “have died hereafter” in any case. Everyone dies at some point. Life passes. Days fly away. Death takes all. “Out, out brief candle!” he remarks. Life is only a “walking shadow” and “a poor player” on the stage for just a little while, a “tale / Told by an idiot, full of sound and fury, / Signifying nothing.” Life is short and meaningless, according to Macbeth, so apparently, the loss of his wife's life is nothing major to him at this point.

We might wonder why Macbeth responds like this. For one thing, his violence and ambition have hardened him. Any compassion he once had has fled with the murders of Duncan and Banquo. Further, Macbeth is completely absorbed with the situation at hand and with his own destiny (so he thinks). He is still positive that he will be victorious, but his confidence will soon be firmly shaken, for shortly after Macbeth receives the news of the queen's death, a messenger tells him that Birnam Wood is advancing on Dunsinane.

Macbeth has a subdued reaction to this news. While many think his reaction shows that he has grown heartless, his resignation may signal something we already know of his character, namely that he is self-absorbed and rarely able to think of anyone but himself. When he says, on hearing of his wife's death, that "she should have died hereafter" (meaning that she would have died anyway), he is reflecting as much about his own fate as he is on her death. This is much the same mood of the famous "Tomorrow and tomorrow and tomorrow" speech, which is a reflection on the uselessness of life—after all his plotting and scheming, Macbeth chiefly regrets that this is happening to him.

A similar reading can be made of his comment on hearing the women wailing over Lady Macbeth's body that he has forgotten how to feel fear. On the one hand, this is his admission that he has become emotionally dead and cannot feel grief as well as fear. But on the other, it is another example of how Macbeth internalizes outside events. It doesn't occur to him that he has some responsibility for her death; instead, he thinks about how he isn't feeling what he thinks he ought to feel. To the end, Macbeth remains weak and indecisive.

Macbeth already knew that the queen was not doing well. In act 5, scene 3, the Doctor tells him that Lady Macbeth is "troubled with thick-coming fancies / That keep her from her rest" (5.3.47-48). Macbeth, in reply, does not really display anything that looks like compassion. He essentially tells the doctor to fix it. He asks,

Canst thou not minister to a mind diseased, . . .
And with some sweet oblivious antidote
Cleanse the stuffed bosom of that perilous stuff
Which weighs upon the heart? (5.3.50-53)

In other words, he just wants the doctor to give Lady Macbeth something to make her feel better, less guilty, to make her "oblivious"—so, perhaps, to make her feel nothing at all; either way, his reply seems to me to lack any real concern for the state of his wife's health. Therefore, when he learns that she is dead, I interpret his words to be the verbal equivalent of a shoulder shrug. He says,

She should have died hereafter.
There would have been a time for such a word. (5.5.20-21)

"Should" seems to me to mean "would" here, and he means that she would have died sometime later regardless, and so the news—or "word"—of her death would have come at that time. However, it seems like all the same to him: now or later, she's dead, and the news is likely not unexpected given that Macbeth was aware of the Doctor's inability to help her.

Of course there is sadness, but I have always read this as Macbeth reacts in a sort of "Well, she would have died sooner or later, but this is really bad timing" attitude.  He is busy preparing for battle with Macduff and his armies.  However, he does take time in his famous "tomorrow, tomorrow, and tomorrow" speech to say that if he and Lady Macbeth had remained honorable, perhaps their deaths could be more dignified than they are destined to be.  They should be receiving rewards and accolades in their old age, but instead, they are mostly alone and unrespected due to what they have done.

Macbeth's reaction to the news that his wife is dead is sadness mixed with regret.  He says, "She should have died hereafter; / There would have been a time for such a word."  He means that he wishes she would have died when he had the time to properly mourn her.  He goes on then to bemoan the current state of his life; how everything has gone badly and life has lost its meaning.  He has become a man driven by his own ambition run amuck as though he were in a car he started but now the gas pedal is stuck fully to the floor and he can't stop it.  The only course of action he can take now is to try to preserve his own life because that is all he has left.  And that, he observes, is pretty much meaningless.