I think you are going off a point here. Humans, are by nature, capable of evil regardless of the situation (I think they are also capable of good).
If you look at nature, it is very cruel by nature as well. Predators typically target the weak and children and not full adults due to them being easier kills. Animals torcher their pray and can be ruthless towards each other.
At the same time, you put them in a healthy, domesticated environment, and they lose a lot of their cruelty.
One issue with Earth and part of the cruelty of man is that their are finite resources. Most wars in the Ancient and Medieval world, for example, were desperate people targeting others to get the necessities of life through the sword. Hence the "Barbarians" we see.