Well, we at least know, according to cognitive science, that belief in at least one god is cognitively natural, not taught. So unless our brain structures significantly change, there will always be belief in the divine. As for whether the belief is justified, true, belief (i.e., knowledge) it takes further argument. So, I guess we should take it seriously because it's not going away, however it manifests itself in different religions, fundamentalist or otherwise. As to these "extreme" forms you mention, it takes looking at the economic and political situations these forms arise from that create particular religious identities. Religion-laden or not, these type of situations will always lead to conflict.

Cognitive Science of Religion:

Scott Atran, In Gods We Trust: The Evolutionary Landscape of Religion

Justin Barrett, Born Believers: The Science of of Children's Religious Belief and Cognitive Science, Religion, and Theology: From Human Minds to Divine Minds

Jesse Bering, The Belief Instinct: The Psychology of Souls, Destiny, and the Meaning of Life

Stewart Guthrie, Faces in the Clouds: A New Theory of Religion

Robert McCauley, Why Religion is Natural and Religion is Not

Ilkka Pyysianen, How Religion Works; Toward a New Cognitive Science of Religion and Supernatural Agents: Why We Believe in Souls, Gods, and Buddhas