Why Antigone Buried Her Brother Despite the Edict

Antigone ignored Creon's decree for two reasons: the gods and love.
Antigone buried Polynices nonetheless because she wished to obey the spiritual laws. The king's laws would always stand beneath the gods' orders. Antigone felt that if she gave Polynices a proper burial, she would be 'right' with the heavens. Since Polynices was of a good lineage, he was granted a proper burial, but after he left the city and Creon was throned, Creon thought he no longer deserved even a hint of respect. Creon would rather deal with the gods' wrath than to see his pride go down the drain.
Another reason Antigone disobeyed Creon was because she was compassionate. For her, she would rather die a painful death than to see her own brother rot in misery. If she would have let this happen, Antigone's honor for herself would be gone, knowing she let her brother die without even trying to save him. Even though Polynices did leave the city and was thought to be a traitor, she still loved him because he was family.