What does Macbeth's reaction to the news of his wife's death say about his state of mind?

Whatever else we might say about the Macbeths earlier in the play, there was no doubt that they were essentially a loving couple. By the end of the play, however, they seem to have "grown apart," as we might say today. Early in the play, Lady Macbeth is remorseless and cruel as she goads her husband into the murder of Duncan and urges him not to feel any guilt for doing so. By the end of the play, she herself has been overcome with guilt, as revealed in the first scene of Act V, when she attempts to wash imaginary blood from her hands while sleepwalking. In the meantime, Macbeth has become a bloody, murderous tyrant, without regard for human life. This development is underscored by this speech, in which he essentially expresses no grief whatsoever at his formerly beloved wife's death. Immediately before receiving the news, he says that after everything he has done, "direness...cannot once start me." He is immune, in short, to horror.
At the same time, Macbeth's speech reveals a sort of grim, existential resignation. Life, he says in reaction to her death, is essentially meaningless:

Life's but a walking shadow, a poor playerThat struts and frets his hour upon the stageAnd then is heard no more. It is a taleTold by an idiot, full of sound and fury,Signifying nothing.

This is perhaps the bleakest passage in all of Shakespeare's plays, and it is obviously Lady Macbeth's passing that has evoked this profound sense of pessimism in her husband. If everything is meaningless--just a march toward "dusty death," then what was the point of everything they have done to seize the throne? In any case, Macbeth's state of mind, already astonishingly bleak, does not improve when he receives the news that Birnam Wood (Malcolm's men concealing themselves with tree boughs) apparently advancing on his castle, thus fulfilling one half of the witches' prophecy.