I had started studying human behaviour and human instincts a few months ago, earlier the question was raised after seeing human behaviour with animals (his prey). Animals hunt their prey for eating, on the other hand, I have seen kids stone pelting at street cats and dogs. Again human approach towards life is more than survival, it is dominating his peer beings. Can I call this behaviour of mankind as a cruel? If so, is it instinctual? I am very sorry for the late response. Hope this clears the question.
It sounds like a philosophical question, but it also has evolutionary roots. All species are instinctually selfish, in that there is a drive to move their genes into the next generation. So misbehaviors (like "cruelty") should be held in check, because they are likely to be punished in a social species. We evolved as a social species that lives in family groups, with a high probability of meeting nearby family groups repeatedly. If you are cruel to your neighbors, you would expect to meet the points of their spears sometime in the future. This expectation of future encounters with neighbors can explain altruism as well. But its a balance. If cruelty to your neighbors is in your long term selfish interests, then you would expect it to be instinctual, just as you might expect kindness to be. Its all about different strategies to send our genes forward. As a side issue, chimps, our close relatives exhibit violence to their neighbors (but bonobos, equally close, are much less violent).