This is an interesting and well-argued paper, but in my opinion, fundamentally flawed.
Let me take as a point of departure the following statement: "Human values are not arbitrary or capricious. Their origins and their continued existence are partly found in the facts of human biology and the interaction of human bodies and minds with their physical and social environments."
If this statement is true in any serious way, then the essence of morality lies in all humans, by virtue of their being human, and is thus inescapable.
How, then, is it possible that we must treat the acts of terrorists "as the immoral and criminal acts that they are." From this, it follows that the terrorists are immoral. But if morality is essential to being human, then it follows that the terrorists are not human.
Since it is evident that the terrorists are human, it is evident that either the terrorists were not immoral, or that morality is not an essential part of being human.