People commonly understand the motives for the September 11th attacks as earthly motives: to resist the U.S. presence in Saudi Arabia, America's support of Israel, or even the existence of freedom. Rarely do we examine the psychological perspective of the hijackers. Why would they would sacrifice their lives to achieve these goals if they won't be alive to experience the results? We often dismiss terrorists as monsters we can't relate to, but this is an abandonment of the scientific method. "Monsters" don't exist; there are only animals that behave in violent ways.
Though the 9/11 hijackers were very religious, it's not correct to say the atrocities were caused by religion. What religion can do is influence one's personal paradigm—one's conception of what is real. If a terrorist believes he might spend eternity with 72 virgins, then this is part of his paradigm. It's what he takes seriously and bases his actions on, actions that are trigged by the same basic biological impulses shared by all humans. His paradigm can help explain his behavior whether or not there's any truth to it.
There's little reason to assume humans would be more peaceful if a cosmic paradigm prevailed, but I do think they would be less willing to sacrifice themselves. After all, self-sacrifice can be rationalized as an investment for someone with a religious paradigm. People have been sacrificing themselves and each other for millennia, and it's tied to belief in an afterlife far too often. Perhaps it's time to examine the results of the religious paradigm for what they are.