How violent, vicious, and morally detestable has mankind been during the (early) course of history?
As to what human beings will or will not do in that regard, I suspect two decisive criteria will always be part of the reckoning: (i) the ratio of costs to benefits, and (ii) technical (non-)feasibility, especially the (non-)existence of a material basis for a particular type of action.
Moral assessments will tend to develop in the wake of acts decided upon according to (i) and (ii).
For this reason, my guess is that there must have been violence and viciousness within and among even the small pre-neolithic tribes of hunters-and-gatherers, while slavery was not yet profitable for lack of a surplus requisite to organise exploitation (conquest, captivity-management, per capita productivity etc). This changed when sedentary agriculture increased productivity in such measure that a division of labour and with it specialists in the exercise of violence and power could be economically sustained.
I have always believed that subject to (i) and (ii) above, the natural trend for mankind is to evolve Structures of Maximal Power (SMP), as I term them, i.e. efficient forms of coercion, which is why the state has come about and will not go away. Inevitable gravitation toward SMP is also the reason why violence and viciousness have a prehistory dating back long before the emergence of the state:
Recent studies have confirmed that mortality from violence is very common in small-scale societies today and in the past. Almost one-third of such people die in raids and fights, and the death rate is twice as high among men as among women. This is a far higher death rate than experienced even in countries worst hit by World War II. Thomas Hobbes's "war of each against all" looks more accurate for humanity in a state of nature than Jean-Jacques Rousseau's "noble savage," though anthropologists today prefer to see a continuum between these extremes.
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