It's natural for humans to fear the unknown, and it's hard-wired instinct for us to fear non-exstience. Religious people tend to fear death less because they have a pretty good idea of what's going to happen to them once they die. The only religious people who fear death a lot are the ones who think they're going to "hell" or some sort of equivalent of that, however, even this involves a certain amount of certainty and at least if you're in hell you "still exist". Atheists, agnostics, etc tend to fear death more because simply not existing and having no "soul" is something that the human mind can't comprehend on an instinctual level. It is easy to comprehend on a RATIONAL level though, and plenty of Internet Tough Guys will read this right now and go "haha, you're talking crap, look at me I have no God and I don't fear death", but I bet if someone were to attack you right now with intent to kill, you would either attack back or defend yourself as a gut-level reaction. Even if you had no reason to live. Survival is hard-wired into our brains, and "fear of dying/nothingness" is part of that hard-wiring, and it exists in every one of us.
