If you stop thinking about it as "difference of religion" and start thinking of it as "differences of belief" you will get a bit further in the reasoning. Religion is just another scapegoat for human nature. Take capitalism vs. communism during the Cold War, even though Religion wasn't a major factor, as an example. You can even do conservatives vs. liberals, Nazi's vs. Jews, but when you really get down to it, it is part of the human condition that separates us by certain beliefs and ideals, and then causes us to clash when someone else with different beliefs is brought into the picture. I could go on at length, but you probably get the idea.
I'd contrast the example mentioned before about evil, or Satan, with my own thoughts. While religion has been used to control by many people, over many years, the thought of right vs. wrong, good vs. evil, and such, isn't by necessity an extension of that. As humans, we have a fascination with white vs. black, immoral vs. moral, good vs. evil, light vs. dark, and on and on. The binary relationship exists in almost every aspect of our lives, and while the definition may be vague, evil does exist, to people, and in their religion. Satan is just a representation of it.