I think it's pretty simple. Antigone IS going to do this: "When I have tried and failed, I shall have failed.... There is no punishment can rob me of my honourable death." She will do what she must to ensure her brother's eternal peace, and she understands she is likely to die in doing so. She sees it as an honorable death, but she is already reasonably certain of her fate if she pursues her intention. She knows it, and the audience knows it, too.
This opening scene also shows Antigone as a woman who will not be stopped. She will not be persuaded or dissuaded. She will not hesitate or waver. She is compelled by a force deeper and more powerful than her very real human sympathies and emotions. She acts out of personal integrity and moral outrage. Being the woman she is, she truly cannot bend before Creon's edict.
Antigone makes specific reference in the opening scene to the prophecies of the doom that would befall the house of Oedipus. She also makes it crystal clear to Ismene that she understands exactly what will happen if she (Antigone) breaks Creon’s interdict and buries Polyneices. Antigone seems to welcome death and certainly does not fear it: “The worst that can befall/Is but to die an honorable death.”
I think that Antigone's zeal is contrasted with Ismene's sense of detachment. I have always been struck by the dichotomy of both sisters as one who lives in accordance to her emotions and the intensity of her feelings while the other is more prudent and possesses a sense of emotional distance. I think that seeing both traits in the opening scene help us understand that one is willing to work within the system to understand justice, while the other believes in the justice that lives in her heart, and is willing to accomplish this by any means necessary.