How does Creon threaten the sentry in Antigone?
The incident with the sentry says a lot about Creon, how intimidating and tyrannical he is. It's not the sentry's fault that someone has decided to defy Creon and bury Polyneices. Yet that doesn't stop Creon from threatening the poor man with death if he doesn't come back and tell him the name of the perpetrator of this outrageous challenge to his kingly authority. If the sentry can't do this, then he'll be held personally responsible for burying Polyneices's corpse. He will then suffer the punishment as set out in Creon's original decree—death.
Creon's threatening of the sentry isn't simply unjust; it's an expression of his chronic insecurity. After all, Creon has just made his first major pronouncement as king of Thebes, and yet already someone's had the audacity to defy him. This sets a dangerous precedent. If Creon doesn't nip this incipient rebellion in the bud, then his throne is in serious jeopardy. It's no wonder he's willing to take out his anger on the sentry and threaten him with death. Even at this early stage, Creon knows that the political stakes are high.
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.")