As someone once said, if there's a jackboot on your throat you don't care if it's worn on the right or the left foot.
I'd say the common element among the extreme right and left is utopianism, and the nihilistic hatred of the real world, and the paranoiac attitude toward it, that results from utopianism. Extremists of either ilk basically believe that justice and liberty can only be realized in the utopia. Human freedom doesn't exist and can't exist in the real world. Any apparent human freedom is a sham, often an intentional one perpetrated by the "conspiracy," the "government," the Jews, etc, and their willing or hapless "shills" and "dupes".
Whether the utopia is the socialist paradise, or the absence of all government, or the perfectly libertarian republic, or the establishment of the global caliphate under sharia law, whether it's a figment of the "right" or of the "left," really doesn't matter, because the utopia will never exist and never has. The common element is that the real world stands in the way of the utopia, and so must be destroyed, and all of its institutions discredited.
Try and tell a Libertarian that their dream world is utopian. About the best you can expect in return is some reference to "Atlas Shrugged".
I do agree with your comments.