Being blindly resentful and being barbaric aren't mutually exclusive.
Of course not, but in the case of the barbaric tribes, we can define them as rustic people with old traditions. Traditions that may be uncivilized to the "civilized" ones.
What I try to say (this is a campus for big debate) is that, in the case of the Falmer, they are just not rational, while in the case of the barbarians (lets take the Forsworm) they fight for an ideal (may be resentment) but they use their reason even in the nonesense of their actions.
The falmer just lost they reason capacity, they are more like angry animals in a permanent state of hate.
Is just my view, of course.