Well, I'd probably say it was their desire for comfort and safety -- IOW, decadence.
The point is, the health of Rome depended on the motivation of its citizens to continue to actively support its conquest and subjugation of the people surrounding it -- whatever it took to do so (e.g., the destruction of Jerusalem in 70 A.D by Titus. Gibbon described his subsequent crucifixion of thousands of Jews in front of the city.)
Obviously this idea of "group needs" makes the whole idea of unalienable rights untenable.
Just to point out -- the reason we're here at all is because the moral implications of "atheist evolution" permit no other conclusion. Once one acknowledges the existence of God, of God's Will, and the revelation and moral implications of God's Will, these logical difficulties evaporate. (Of course, human nature being what it is, moral misbehavior does not disappear.)