This gets me mad because they have no reason at all to /hate/ it, they just do
Religion bothers me deeply because it is the an introduction to circular logic, rhetorical stagnation, and generally encourages close-mindedness. It can very quickly turn people, often of very similar mind, against each-other for no good reason. It often takes away from the appreciation of the beauty of step-by-step formation of a delicately balanced world.
Religion can and does kill people.
However, this isn't all religions. Buddhism is pretty cool. And other than those points, I wouldn't have any real reason to dislike religion. It genuinely can be helpful to some people.