How we cite our quotes:
MACDUFF Boundless intemperance In nature is a tyranny; it hath been The untimely emptying of the happy throne And fall of many kings. (4.3.7)
Note that the problem isn't the king's death—it's that the king's death was "untimely," thanks to Macbeth's boundless intemperance. In other words, Macbeth simply didn't have the patience. Maybe if he'd waited he would have become king in due time—and not at the wrong time.
MACBETH She should have died hereafter; There would have been a time for such a word. To-morrow, and to-morrow, and to-morrow, Creeps in this petty pace from day to day To the last syllable of recorded time, And all our yesterdays have lighted fools The way to dusty death. […] (5.5.2)
You can interpret this speech six ways to Sunday, but it seems pretty clear that, however he feels about his wife, Macbeth is pretty sure that he no longer has a future.
MACDUFF Despair thy charm; And let the angel whom thou still hast served Tell thee, Macduff was from his mother's womb Untimely ripp'd. (5.10.3)
Hey, "untimely"! We just saw it in the "untimely emptying of the happy throne" (4.3.7), so there's definitely something going on with that word. Is an untimely birth the only antidote to an untimely death?