Esav hated Ya`akov because of the blessing with which his father blessed him. Esav said in his heart, *The days of mourning for my father are at hand. Then I will kill my brother Ya`akov.*
When Yosef's brothers saw that their father was dead, they said, *It may be that Yosef will hate us, and will fully pay us back for all of the evil which we did to him.*
It was now two days before the feast of the Pesach and the matzah, and the chief Kohanim and the scribes sought how they might seize him by deception, and kill him.
When it was day, some of the Judeans banded together, and bound themselves under a curse, saying that they would neither eat nor drink until they had killed Sha'ul.