Macbeth has been so fixated on his ambition and power that he has missed what really matters - or, in a more nihilistic interpretation, he never realised that nothing really matters. His ambition can't live on past his death, and nor can his power. His crisis is caused by the undeniable truth of his own mortality, which nothing can contend with. For all his "sound and fury" - his violence and painful guilt - he has accomplished nothing everlasting: it signifies "nothing".