Is Dictatorship the Greatest Sin in Islam?

Yes, according to the Ayatollah Khomeini.

Whatever else one wants to say about the Ayatollah Khomeini, it seems hard to escape the conclusion that he made the Iranian revolution—and, by extension, the Iranian post-revolutionary state—into an idol. In so doing, it wouldn’t be too far off the mark to say that Khomeini committed a form of idolatry. Whether he might be punished for this in another life is not for us to say. It is only God, if you share my premise that God exists, who can make such a judgment. Of course, if God doesn’t exist, then Khomeini doesn’t have much to worry about, regardless. (So if you want Khomeini to at least potentially suffer for his sins, it is probably in your interest to believe in the very God that Khomeini believed in).