I think that there are many dangerous beliefs. Having said that, what muslims, atheists, and Jehovah's Witnesses all have in common is that they reject Jesus Christ as their Mediator. Now that IS very dangerous.
Believing that someone can survive Judgment day without a Mediator is far more dangerous than whether or not to entertain thoughts of an afterlife. While the Muslim and JW positions are completely incompatible with the impending event of the DOJ, at least the atheist position is consistent, even if incredulous.
In other words, the Muslim and JW agree that there is a DOJ, but are at a complete loss as how to explain how their sins are forgiven (a.) outside of the New Covenant and (b.) without Jesus as their Mediator and absorbing the death blow, in their place, to satisfy the Law.
Atheists must somehow convince themselves that everything either self exists, or came from nothing. It is very difficult to understand how they can be so critical against the self-existence of God but at the same time entertain the exact same thing about the universe. Huge holes in all these beliefs.
Judgment is the great answer to the "problem of evil". Few seriously consider the ramifications of that event. Whether they be cultist or atheist, they have the same flippant attitude about it.