Yosef dreamed of dominion over his brothers. They, obviously, did not want to be subjected to his dominion. What is the only fail-safe way to assure that Yosef does not achieve his dreams? Killing him.
I think this is the simplest reading of the text. In 37:19 they say
הנה בעל החלמות הלזה בא
Behold this dreamer is coming
Then they suggest killing him and they say
ונראה מה יהיו חלמותיו
And we will see what will become of his dreams
I think this is best represented by Ralbag's commentary.
והסכימו ביניהם שיהרגוהו וישליכוהו באחד הבורות ויאמר ליעקב שחיה רעה
אכלתהו ואז יראו מה יהיו חלומותיו ר"ל שאז יתברר להם שלא יתקיימו
חלומותיו
And they agreed among themselves that they would kill him and cast him
into one of the pits and tell Yaakov that an evil animal devoured him,
and then they would see what would become of his dreams. Meaning to
say, that then it would be certain to them that his dreams would not
be fulfilled.
In fact later in Parshas Mikeitz (התועלת השמיני, The Eighth Lesson) Ralbag mentions that the lesson of this incident is that man should not attempt to thwart the Divine plan. The plan was for Yosef to rule over them, and not only did selling him not thwart this plan, it was the very catalyst that brought the plan to fruition. He compares this to the legend of the founding of Rome, wherein the father of Remus and Romulus dreamed of being overthrown by his children. He banished them to be left to die in the forest, but not only did this not thwart the plan, it enabled it because the children were raised in the forest by a wolf and eventually overthrew their father and founded Rome.
The point here is that the only true way to thwart such a plan is to kill the person involved in it. For as long as the person walks the Earth the plan can still be fulfilled. Thus, the brothers had no choice but to kill Yosef; anything less would not guarantee saving them from his dreams of dominion over them. Indeed once they were convinced by Reuven and Yehuda to not actually kill him but to try to thwart his plan in another way, we see that they did not succeed.
(Ralbag might argue that even killing Yosef would not thwart the Divine plan, either because God could prevent them from killing Yosef, or because He could somehow fulfill it after Yosef is dead.)
From the short account of the attempted killing and then sale of Yosef in Genesis Chapter 37 verses 18-36 there is no textual evidence of nefarious intent or hatred towards Yosef. They were simply doing what they had to do to prevent him from achieving his dreams of dominion over them.
