The key issue in Creon's decision to wall Antigone up in a tomb rather than kill her was a concern over ritual pollution. The Theban story is one that begins with Laius violating the rules of hospitality by raping the son of his host Pelops. This brought down the anger of the gods on Laius and all his descendants. Oedipus's efforts to discover the source of the plague in Oedipus Rex and Creon's attempt to bring peace and justice and an end to fratricidal wars are all attempts to propitiate the gods in some way and end the ritual pollution that is harming not just the immediate relatives and descendants of Laius but the city as a whole.
By walling Antigone up, Creon thinks he avoids actually being responsible for killing her. If she dies it will be the will of the gods. He thus hopes to avoid the endless cycle of ritual pollution and divine vengeance. The cave (which one could compare to the cave where the Delphic oracle is situated) is thus symbolic of Creon's attempts to evade divine vengeance, attempts that we see fail as his son and wife commit suicide. Thus the cave can be a symbol of the futility of trying to evade fate.
There seems to be something of a motif of entombment running through this play. Firstly, there is the central irony of Antigone being entombed alive, when what started the conflict in the play in the first place was Creon's inability to entomb Polyneices, Antigone's brother, as he should have. Creon seems to want to entomb those who are alive rather than those who are dead. What is also significant about Antigone's fate is the way that she refers to the tomb:
Oh tomb, my bridal-bed--my house, my prison
cut in the hollow rock, my everlasting watch!
I'll soon be there, soon embrace my own,
the great growing family of our dead
Perspephone has received among her ghosts.
For Antigone therefore, she seems to sense that the tomb will be a place not of tragedy in some ways, but even her "bridal-bed," as it will represent her unification with her lover and also the rest of her dead family. This is something that she greatly looks forward to as she thinks about the prospect of her entombment and joins the "great growing family of our dead" who are now in the underworld, having died. The symbol of the entombment therefore works on a number of different levels in the text to both highlight Creon's mistake in the play but also to demonstrate Antigone's thoughts and feelings about death.