What is Creon's main concern at the beginning of the play?
Creon's main concern is to govern Thebes with authority, but also with justice. Therefore, he is in favor of following the law to the very letter. Additionally, he is concerned with asserting himself as king (he is new to the position) and coming across to his people as authoritatively decisive. This is why he makes an almost immediate decree against the treason that has taken place in Polyneices' attack against his brother and the city.
As soon as his decree is broken, Creon again makes a hasty decision which he announces with a firm sense of finality before weighing the full circumstances. Rather than asking what might be best for the entire state, he reacts with narrowmindedness and becomes consumed the principle of the law rather than the situation as a whole.