Creon's motivation is to uphold the state above all else. The rule of law supersedes all other considerations. Polyneices, though a member of his family, broke the law when he rebelled. Therefore Polyneices must accept the consequences of dishonor that come with action taken against the state.
His conflict, however, is with Antigone, who holds to a higher law. Above the rule of law comes the rule of love. That is a law that supersedes the state. She wants an honorable burial for Polyneices because he is family. He was family before he was a traitor.
Creon sees this intrusion of a higher law as a threat, not just to his own power or his government, but to civilization as a whole. Too late he realizes it is the reverse. Love before law.
Creon is in many ways a tragic hero in this play. Although modern audiences tend to sympathize with Antigone, Creon is, in some ways the more complex and interesting character.
The first motivation we can ascribe to him is patriotism. As he is presented in both Antigone and Oedipus Rex, he bears a deep love for his city and is concerned and pained by the misfortunes his city has suffered, first the plague brought about by the curse on Laius and his descendants, and then the fratricidal wars between Eteocles and Polyneices. While he is concerned about family, part of what makes him a noble character is that he is willing to put his belief in law and his moral code above even his personal life and family, stating:
For if any, being supreme guide of the state, cleaves not to the best counsels, but, through some fear, keeps his lips locked, I hold, and have ever held, him most base; and if any makes a friend of more account than his fatherland, that man hath no place in my regard.
Although he appears to us harsh and inflexible, and is perhaps too prone to favoring his own judgments rather than listening to others, his basic motive is to restore Thebes to peace and prosperity after decades of disasters and he thinks that strong leadership is the most effective way to do that.