why is creon's pronouncement to kill the sentry an act of injustice?

This is a case of what people today call "shooting the messenger". The sentry is merely reporting what he has seen. Apparently someone has given proper burial rites to Polynices’ corpse. Creon flies into a rage and accuses the sentries, this one in particular, of taking bribes from his enemies to defy his edict. Creon threatens to murder the sentry if a suitable perpetrator isn't found. This is pretty much a textbook case of injustice. Look up injustice in the dictionary and you will find Creon's picture! Okay that's an old joke and not really that funny. The point is that Creon's rage, and perhaps guilt, over his edict is clouding his judgment. He has all kinds of emotional issues attached to his edict of non-burial. When he feels challenged he goes after the person merely delivering the news rather than seeking the real cause of his distemper.