The other people in her family that had died, such as her parents and her brother Eteocles, had all been peacefully buried. During this time, someone not being properly buried was a terrible thing and it would be shamed. This is why when Antigone asks Ismene to join her, because it is the just what you do to respect your family, she thinks Ismene is foolish for not joining to put her brother to rest. If Antigone doesn't bury him she thinks she will not have honored her brother properly and Antigone believes that the worst death is death without honor.
In Greece it was strongly believed that if not buried, a person’s soul will be left forever to roam the earth with no place to go and move on to. It was a terrible thing to happen to anyone. So, when it happens to Antigones brother she feels the need to help him and would feel guilty if she didn’t. Also, she wants Polynices to have to same peaceful and respectful funeral as the rest of her family had had. If she hadn’t buried him, he would be laughed at and Antigone wants to be courageous and assist him with moving