While classical tragedy generally involved heroic people in simply awful situations (often of their own making), modern tragedy places everyday people in similar quagmires in a contemporary setting. The tragedy of "Oedipus Rex" involves a young prince involved in a prophecy that foretold he would kill his father and marry his mother, and when that improbable scenario comes true, the king needlessly blinds himself, abdicating his throne and leaving Thebes in chaos.
In the modern tragedy "Death of a Salesman," the main character is just a salesman, but his choices, when he comes to realize that he can no longer support his family, are writ just as large on the stage, as he needlessly commits suicide just a matter of weeks from paying off the mortgage, leaving his family in ruins.
While such tragic characters as Agamemnon, Antigone, Othello and Julius Caesar all stand tall in the pantheon of drama, their stories are not any grander in scale than the trials that modern people face over the course over their lives. Agamemnon needs to sacrifice a virgin to go off and fight his war, and he makes the awful mistake of choosing his daughter. In the tragic ending to Rabbit Angstrom's marriage in "Rabbit at Rest," his sensual greed leads him to make love to his daughter-in-law, an act that he can only follow by fleeing to Florida. The greed, covetousness and rage that lie within humans are the stuff of tragedy, whether one is a monarch or a mailman.