Originally Answered: When does religion turn from a force for good into a force for evil?
The problem with religion is that it is inflexible, while also being utterly accepted.
See, in places like America, people love to talk about gray areas. We know right and wrong, mainly, but we have the freedom to decide shades of right and wrong, making personal judgments and leaving it to majority vote what should be allowed or disallowed, even if it means allowing something bad in order to, potentially, acquire some good thing that cannot otherwise be obtained.
In other places, perhaps under totalitarian rule, an individual makes rigid rules that people must obey. In this scenario, people know what is right and wrong, but they have no time for gray areas, since opinions or beliefs don;t matter. They must either do right or wrong, in accordance with their ruler's beliefs (or desires) on pain of punishment.
With religious rule, the first impression is that it is best. It is rigid, so there are no gray areas, yet everybody agrees that they are doing the right thing anyway; so it's all good. Nobody has a problem with a strict ruler, since there is no cause for rebellion. But the difference between a religious ruler and a totalitarian ruler, as history has shown, always skews and blends. It is only a matter of time before ambitious men with the power to enforce right and wrong realize that they can manipulate a throng of faithful, willing people who are entirely sure of the correctness of their actions. And before you know it, generations can go by with people living and dying by a corrupted system that they have mistaken for the ideals of their ancestors.
And when the revolution finally comes, as people realize that their religious fervor is for a different cause than what they formerly believed, hindsight, very suddenly, displays a world of evil done in the name of something that everybody thought was good.