There is a hot way to kill and there is a cold way to kill. I think the Nazis epitomise a cold and calculated killing apparatus, with the act of keeping records of numbers of those killed and the neat recording of their scientific experiments being the real horror for which many thinkers mention the Nazis. As Hannah Arendt once said, the horror consists in the banality of evil, which shows no honour, involves no real oath, in which the individual defeats another individual in a physically waged battle, bloody though it may be.

What contemporary man understands by barbarism, however, means the bloody confrontation, the stabbing or beheading, the impaling and spearing, and understands it as killing for the pure lust of murder.

Here we find a paradox, because in order to kill another warrior as a warrior, murderousness is indeed a state of mind into which one must bring oneself. Otherwise he could not go into battle.

But since the lust for murder is not only on his side but also on the side of his adversary, the outcome of the battle is uncertain and so anyone can lose his life. If a warrior emerges victorious and alive, the joy may be not malice but the logical consequence of a battle fairly fought, and where the so called barbarian believed in deities he had to believe in order to remain sane and have a divine justification for killing another, this is the most ideal justification, for there is no other superior to it.

Since it is a fact that warfare is as much a part of man as his capacity for cooperation and clever diplomacy, it cannot be ignored. Creation and destruction seems to me to be a cosmic principle.

It is impossible for us modern humans to think and feel this way because we are not warrior peoples but city dwellers.

Modern methods of killing, on the other hand, are impersonal, distant and enable the death of masses in one fell swoop. Since modern man is not a warrior, and he sees shredded human corpses in the war zone, he becomes traumatised and injured in mind and soul. He is physically and mentally no match for a hail of bombs, the sheer overwhelming power and volume of the machines over his vulnerable body can give him nothing but horror. He can see no sense in this form of battle since, indeed, it is senseless.

It is utter madness for armed groups to gather against unarmed ones and then imprison them en masse and kill them with mass extermination methods (gas chambers, mass shootings). Anyone who does this is worse than a barbarian, yes.
Honour would dictate that a real fight be organised where the conditions are similar on both sides. But to take prisoners without there having been any battle at all, without where the prisoners are a result of victory, is simply abominable.

As repugnant as the methods of former cultures seem to us today, we are not entitled to condemn them because they had a local cultural background, such as your example of cannibalism. Whereby I think that cannibalism may never really have been a mass culture, rather a kind of desperate affair.
The Roman Empire, on the other hand, and what I think I know of it, is another matter. The "Brot und Spiele" in the arenas seem to mirror a cultural decay.