The question of how to judge the motive(s) and sincerity of Korach's rebellion I think depends in large measure on the machlokes between Ramban and Ibn Ezra as to when the rebellion took place. If one takes the view of Ibn Ezra that the rebellion took place immediately after the first born were replaced by Levi'im and Kohanim -- i.e. chronologically the parsha of Korach occurred immediately after cheit ha'eigel -- then perhaps there was a certain sincere religious motivation to the revolt. Coming so close to ma'mad Har Sinai where the entire nation was told that they were a "mamleches kohanim," the displacement of the bechorim and the creation of a tiered system of Levi'im and Kohanim as exclusively responsible for avodah, with Aharon, the very person who crafter the eigel, as kohen gadol, would create an understandable feeling of frustration. That is not to say that Korach was correct -- simply that the position of those involved becomes more understandable and that perhaps there was a degree of idealism that motivated them.
According to Ramban, the story of Korach occurred chronologically after the meraglim episode. Korach waited to act until Moshe's power was weakest. The people had just suffered the setbacks of the punishment of the misonenim, kovros ha'ta'avah, the meraglim and the decree that they would not enter Eretz Yisrael, they had just heard the prophecy of Eldad and Meidad that Moshe would die in the desert with them -- the time was ripe of revolt. Perhaps I'm wrong, but I don't think an idealist concerned only with religious reform would engage in such political calculations before making his move.