All in the Name of God
Had Yosef been a bit more righteous in his brothers' eyes, they may have conceded to his destiny as either the 'leader' or the 'chosen' son. However, their perception of Yosef's character troubled them. In their eyes, Yosef was a slanderer:
"And Yosef brought bad reports ('diba ra'ah') of his brothers to his father." (See 37:2)
The brothers, aware of the challenges facing God's special Nation, recognized the need for exemplary leadership. Could Yosef possibly assume this role? To the brothers, the mere thought of 'Yosef the Slanderer' becoming the leader was horrific. From their perspective, it was simply unthinkable that Yosef could assume the leadership of a nation destined by God to be characterized by "tzedek u'mishpat" (see 18:19). For the sake of "klal Yisrael," they conclude: Yosef must be weeded out!
Hence, the brothers faced a predicament similar to that of Rivka in the previous generation. Just as Rivka had realized that Yitzchak was mistaken in his favoring of Eisav, so too the brothers realize Yaakov's mistake in favoring Yosef. Just as Rivka resorted to 'trickery' to ensure that the proper son would be blessed, so too the brothers decide to use 'trickery' to ensure that Yosef would not be appointed their leader. Considering that the entire fate of "Am Yisrael" was at stake, the brothers allow themselves to 'bend the rules' a bit, so as to secure the nation's future.
An ideal opportunity (for the brothers) arises when Yosef arrives at Dotan to visit them. In order to dispose of this menace, they plot first to kill him. Later they opt to sell him off to a distant land. In either case, their stated goal is to make sure that Yosef is removed from the Divine family (see 37:20 - "v'nireh mah yihyu chalomotav"). Out of respect and concern for their father, lest he fret and worry about his 'missing' son for the rest of his life, they will dip Yosef's coat in blood so that Yaakov will think that he was truly dead. Hopefully, their father will finally realize that Yosef was "nidcheh" (rejected), and now Am Yisrael can continue to develop in the proper fashion. (See Board #2.)
Thus, based on the theme of Sefer Breishit, the brothers' plot to dispose of Yosef, though inexcusable, is understandable. It is not simply out of petty jealousy that they want to kill Yosef, but rather out of a 'sincere' concern for the future of Am Yisrael.