In Greek drama, the protagonist is the tragic hero. He, or she, will have a character trait, or flaw, that will bring on his downfall–possibly his death.
For instance, in Oedipus Rex, it is the hero’s relentless search for truth at any price that brings about his uncovering the truth and stabbing out his own eyes. OR believed in the truth at any price–and that was his flaw.
As for your particular play, ask yourself: what character trait does the tragic figure have that drives the story forward and brings about the tragic end?
You should be able to see that one trait expressing itself with each new plot development. And each development is consistly pointing to an end of the play.
Note that plot is driven by CHARACTER.

After reading the Sparknotes summary:
Antigone insists on burying her brother Polynices. To the Greeks, the burying of relatives was very important. Her obstinacy at all costs is what leads to her death, the death of Haemon and Euridice. Creon is left all alone.
It is this dogged determination to bury her brother despite Creon’s order to leave his body to rot that drives the story forward and leads to the inevitable conclusion: the tragic ending.
And since Creon is left alone and isolated, he too is a victim of his own particular tragic flaw–the insistance that sacred burial rites not be carried out.