I think honestly many of those folks haven't really thought about it, but are just parroting something they heard from their clergy or something. I've had conversations with some devout people who were surprised to find I'm atheist. I like to think they change their thinking after realizing I'm nice, ethical and moral, without a belief in god/s. I do feel that as more of us nice atheists come out and create awareness, that myth about atheists being evil will evaporate.
You nailed it there. So few people are introspective enough to examine what they believe and why. Its so easy just to go with whatever they heard from the pulpit without any analysis. Thinking is hard! Religion is easy - they actually encourage you not to question or think. Its paradise for the lazy brains.