The Portuguese brought 'Latin and Christian culture' to Brazil, this can be considered 'founding a civilization'. But they were extremely cruel with the slave trade and rape of women. The fact that the Romans - and others - founded civilizations does not change all the cruelty that was practiced. Unfortunately, cruelty was universally practiced. As a descendant of Portuguese I do not feel that I am in the position of judging the Vikings as 'especially cruel'. I have no admiration for the Vikings or the Portuguese 'conquerors'. Nor does it make sense to compare who was more or less cruel. I don't understand why almost everything here needs to turn into a 'Southern Europe versus Northern Europe' debate. Believe me: in the American continent both northern and southern Europe have left their bloody marks.
The "cruelty thing" is a relative one and not restricted to any sort of rule or structure. The problem of post WW2 is that a narrative was established in which "the state" and "the powerful" being always guilty, and the assignment of guilt shifted to the guilt of "civilised men", when in fact many "cruelties" were just a reaction to a provocation and cruelty which happened before from the other side, the resistance to the "powerful". Who is "powerful" and who is "suppressed" is also up to everyone's definition and can change with a blink of an eye of history.
Life, competition, conflict, warfare is cruel in general. That's just how it is. People can ignore this or there can be cultural mechanisms to temper it, to create a less cruel and brutal environment, but by default, that's the world as it is, even with or without humans. With states and law cruelty and brutality got drastically reduced. But for being able to exert this kind of hierarchic control, which reduces brutality and cruelty, you need to establish a power structure first. And how do you think any tribal people accept that transition, in which their local leaders and chiefs, their long held traditions being cracked? Some may by seeing material and social advantages, but others won't, they will resist. And if they resist with cruelty and brutality, which they often do, the state will try to defend its citizens and soldiers as well, or even take revenge, pure revenge for its own people's suffering. Its simple as that.
Many "assymetrical wars" lead to provocations on both sides, in which brutality and cruelty escalates. Like if you find your comrades mutilated and tortured to death, no matter when, no matter where, no matter for which reasons, the most natural human thing to do is to go out for revenge. That's not related to any state or tribe, it doesn't matter on which sociocultural or civilisational level you stand, you just do it, that's normal human behaviour. Only if the state uses its soldiers as a disciplined war machine and wants to keep up civility and order at all costs, even by punishing his own warriors or soldiers, for doing so, or if the soldiers got an extreme ethos of "give the other cheek", it won't end that way. Otherwise it always does, and its just human behaviour.
The tribal warriors did the same or more if getting the chance to, if experiencing something similar. The main difference in the case of Rome was the magnitude, the sheer scale. They did exploit and rape other people on a grand scale in some cases. The big problem of states and civilisations is that they record their doings, so now, in different times, with a different mindset, people can read their "crimes", while what the people with no written records, with no recorded history is lost. They just "did their thing" and nobody will ever know, unless a archaeological surprise comes up like in Iberia, Austria or Eastern Germany, with Neolithic mass graves pointing to executed prisoners or human sacrifices. If there is no record, there is no way to know what exactly happened, but the ancient DNA in itself gives a lot of hints, because people don't just disappear.
If anyone wants to prevent cruelty and brutality from happening on a grander scale, one has to establish an effective state, with a population on a high civilisational level, common cultural identity and avoid war or even violent conflicts as such. Once a real war, and I mean more than just "policing", but an essential and pivotal war starts, in which its about all or nothing for a people, everything will go down the drain quite swiftly. There is no clean war and the more decisive and elementary the conflict is, the more brutal and cruel it will become.
To make it concrete, in a lot of cases Caesar did first subdue a people in Gallia rather peacefully, left them in peace and relative freedom, but under Roman rule. He had no intention or need to brutalise them and left only a small garrison there. But as soon as his position looked weak, the tribe rose up and truly massacred the garrison. He came back, defeated them, they pleaded for mercy and probably they got it once more. The next year, the Romans were in a dire situation, the same thing happened, and they showed no mercy with any Roman, including civilians, they could grab. Well, Caesar did make it again, came back and this time he massacred the whole tribe and sold the remains into slavery... By this way he not only took revenge, but also made an example of them. The next tribe or city would just surrender before it went to the bitter end. The death toll in Gallia for example can be largely attributed to a series of brutal uprisings, in which regional leaders and tribes broke their vow numerous times, whenever they thought the chances for an uprising would be good. One could ask what the Romans did there in the first place, but that's a different question in my opinion to whether or not the Romans were more brutal. Especially since all other people with some sort of success, like the Celts themselves, expanded whenever getting the chance to as well.
Or just read what the British Celts under Boudica did with Roman citizens, including females and children. In most of these conflicts, with a very few exceptions, the cruely and brutality was reciprocal, but what many people remembered or point out more is what a state with historical records did. Probably one of the few exceptions were the Assyrians and Mongols among others, these two were really brutal and cruel far beyond the "reciprocity rate".
