Again - I'm not arguing necessarily that any of them started out that way, in fact - I'm willing to bet that very few (looking at you, Mormons) actually were. Most religion (in my humble opinion) just stems from folks trying to make sense of an unfathomable universe using what tools are available to them at the time. But once you have the religion, and you have holy men/women who have the ability to excersize some form of power over their flock, you'll inevitably find corrupt people flocking to those positions, as they do in every position of power. Then over time they'll carve out more power for themselves and more authority, find ways of extracting influence and power from their positions until soon you've got "holy men" living in palaces with the authority of kings.

It's just human nature for positions of power to eventually become corrupted to some degree, and positions of religious authority offer an unparalleled lever in which to move the masses, which only serves to make it more attractive to would-be tyrants