The sentry brings news to Creon that Creon's newest (and probably first) order as King of Thebes has been defied: someone has tried to bury the traitor Polynieces. Creon demands that the sentry find out who the perpetrator of this crime is. Otherwise, Creon will hold the sentry responsible and punish the sentry according to the punishment set forth in Creon's decree: the sentry will be sentenced to death. This is most likely why the sentry argued with his fellow soldier as to who would tell Creon, and why the sentry takes so long explaining what happened.
(One of the first instances of "don't kill the messenger.")