Seth is most famous for the fratricide of his brother Osiris and the attempted murder of his brother’s son, Horus. Horus survived though and avenged his father’s death by ruling all of Egypt and exiling Seth to the desert for all time. The decision to banish Seth came from a counsel of the gods, ruled by Re. While most of the gods agreed with Horus and his mother Isis that Osiris’ son was the rightful heir to the throne of Egypt, Re disagreed. He believed that Horus was too young to hold such a powerful position. Thus, the trial was stalemated for many years. Only the cunning of Isis could bring the case to an end.

Using her magic, Isis transformed herself into a beautiful young woman. Seth saw her with tears streaming down her face and asked what the matter was. Isis told a story not unlike the situation of herself and Horus, where an evil man had killed her husband and was trying to steal her family’s flocks. Seth became angry at her plight and insisted that the evil man be destroyed and that the young woman’s son should inherit the family’s estate. By his own words, Seth condemned himself, and lost the throne of Egypt.

http://egyptianmyths.net/seth.htm