Tragic heroes, as traditionally defined, always have far to fall. That is to say they must be high up the social scale and have lots to lose, whether it's wealth, power, or social prestige. Macbeth is about as high up the ladder as it's possible to get in the kingdom of Scotland. But not quite. Though showered with honors by a grateful king, there's one title that Macbeth has not yet attained. That's the kingship itself.
Inspired by the three witches's prophecies, and egged on by his super-ambitious wife, Macbeth develops an overriding ambition to be king. His ambition can only be achieved by having the existing king, Duncan, murdered. This is a prime example of what the ancient Greeks called hubris, an excessive pride or overweening ambition.
In Ancient Greek theater, it was hubris that most frequently led the high-born to disaster, as in the case of Oedipus in Oedipus Rex. And it's the same with Macbeth. Although he ascends to the throne of Scotland after killing Duncan, his subsequent descent into blood-thirsty barbarism culminates in his own death. Macbeth overreached, and in doing so, brought about his own dramatic downfall.
Much the same could be said of Dr. Faustus. Like Macbeth, he appeared to have it all, but also like Macbeth, it wasn't enough for him. Despite his reputation as a great scholar and man of letters Dr. Faustus is profoundly dissatisfied with life. He wants power, the kind of power that only the Devil can provide. So he summons the Devil's acolyte Mephistopheles and offers to surrender his soul in exchange for twenty-four years of power.
As with Macbeth, Faustus's hubris leads directly to his downfall. Having made such a Devil's bargain, it's inevitable that his soul is in mortal danger. Though Faustus is constantly aware that one day his soul will go to hell, he doesn't ask for God's forgiveness. Instead, as the hour of doom approaches, he throws himself at the mercy of the Devil. Like Macbeth, Faustus hasn't really learned his lesson. The consequences are disastrous.