“Come now therefore, and let us slay him, and cast him into some pit, and we will say, Some evil beast hath devoured him: and we shall see what will become of his dreams.” Gen. 37:20
Those dreams that Joseph had really bothered his brothers. They were afraid they would end up being true. So if they could kill him it would put an end to his dreams. You know, you have to be careful who you share your dreams and promises from God with. There are plenty of skeptics out there that will put you down.