As far as I see it, religion actually had humble beginnings. It was simply a way for primitive peoples to cope with the world they found themselves in without any way to explain it. Natural occurrences such as lightning, earthquakes, hurricanes, tornadoes, et. al., were terrifying to them and they probably couldn't comprehend that they were, in fact, natural happenings and attributed it to some unseen intelligent force, or god(s), that was angry with them for some reason. Animal attacks were thought to be punishment as well instead of it just being an hungry animal looking for a meal.
However, it wasn't long before, in order to appease these unseen gods, that they began to offer sacrifices, which ultimately included humans. They eventually began coercing others into "believing" the way they did, ostracizing them if they didn't.
Religion didn't start out as evil. But, unfortunately, that is, for the most part, what it has become. Kali demonstrates that not all adherents to religion are evil, as does Sandman and a few others around here. I have several very good friends that are quite religious and they are very sincere in their beliefs. But I view religion as an antiquated way of thinking and modern science has gone a long way in explaining the way things work without the need for gods. Yet the religious leaders insist that the science is wrong in the face of overwhelming evidence to the contrary just to retain their control and power of that zealous congregation you speak of, B-Man.
And that is the real evil in this.