They were a slave society and were particularly harsh on slaves, and particularly rebellious slaves. The cruelty of a society where crucifixion (scaleable, low labour cost, torturing to death) was a standard punishment is mind boggling.
Then there was all the rape (of slaves, it was fine, even children) and other cruelties.
Civilized does not necessarily mean nice.
From Tacitus:
It was subsequently decided that Sejanus’ remaining children should be executed, though the mass anger had abated and the majority had been placated by the previous deaths. They were therefore carried off to prison, the boy (Capito Aelianus) aware of his fate, the girl (Junilla) so innocent that she kept asking what she had done wrong and where they were taking her, that she would not do it again and surely could be punished with a trivial beating. Historians of the time say that as it was thought to be unheard-of for capital punishment to be inflicted on a virgin, she was violated by the executioner, the noose beside her; both children were then strangled and their young bodies thrown down the Gemonian Stairs.
The rape by executioners was a uniquely Roman twist AFAIK, but executing children to punish parents was not. For example, it was done to a traitor's family in 19th century Sri Lanka (repugnance at this helped the British overthrow the last remaining independent kingdom on the island). Probably many other places did the same.
> He said he had been a highly regarded member of the force, and had so "impressed my superiors" that, at 18, "I was given the 'honor' to temporarily marry young girls before they were sentenced to death." In the Islamic Republic it is illegal to execute a young woman, regardless of her crime, if she is a virgin, he explained. Therefore a "wedding" ceremony is conducted the night before the execution: The young girl is forced to have sexual intercourse with a prison guard - essentially raped by her "husband." "I regret that, even though the marriages were legal," he said. Why the regret, if the marriages were "legal?" "Because," he went on, "I could tell that the girls were more afraid of their 'wedding' night than of the execution that awaited them in the morning.
https://m.jpost.com/iranian-threat/news/i-wed-iranian-girls-...
Records from other groups are pretty rare and largely coloured by those later ones who came after them and can put lot of blame on them. While very well scrubbing their own wrong doings.
Fast forward more than a thousand years to the multiple crusades.
Today we can say that Christianity has mostly calmed down, but it took basically 2000 years.
For example, the crusades are often presented as an unprovoked war, but started in response to an appeal for help to hold back the expanding Turkish empire, and aimed to conquer (arguably liberate) lands that had been subject to fairly recent invasion by the Arab and Turkish Empires and still had large (maybe majority?) Christian populations.
It took place in the context of Arab occupation of large chunks of Europe (most of Spain, Mediterranean islands including Sicily) and invasion of more (southern France, southern Italy).
I would say that in many ways Christianity has done the opposite of "calm down" in the last few hundred years. The adoption of the (originally Arab Muslim) argument of "the curse of Ham" to justify racism (and slavery) in early modern times (mostly by people with a financial incentive to believe it) was one nasty development, and the(still continuing) rise of Biblical literalism in the last two hundred years or so is another disaster.
If you are interested in a real history of Christianity in the West then Dominion by Tom Holland is excellent.