In Macbeth, how is there a smooth transition from the real world to the world beyond?

Initially, Macbeth's political ambitions seem perfectly normal for someone of his time and class. Yes, there's something deeply treacherous and disturbing about his betrayal of Duncan; but there's nothing out of the ordinary about it. All that changes as the play progresses, however. Once Macbeth's safely established on the throne, his actions go beyond the pale, crossing over into outright evil. His cruel, bloodthirsty behavior towards Banquo and Macduff's family is utterly senseless. In ordering their brutal murders, Macbeth has made a disturbingly smooth transition from the natural to the supernatural world.
But then, this shouldn't really surprise us. For the Weird Sisters' prophecy pertains to both worlds, admitting of interpretations that are consistent with the natural and the supernatural. As Macbeth's tyranny becomes ever more bloody and unhinged, mere earthly ambitions give way, and he takes his stand unequivocally with the forces of darkness, making it easier for his enemies to express their opposition in terms of a struggle between good and evil, instead of as a bid for political power.