I'd like to think we are built to be kind but I really doubt it.
We have to teach children to be kind. We don't have to teach them to be cruel or unkind.
Is this a joke? Or do you really believe this?
Having watched my daughter grow up and sent many nephews/nieces, children of friends etc. it is a constant watch to ensure that they do know and behave in a kindly matter. It does not take much to have their display behaviour that is unkind or cruel.
Have you asked yourself what behaviors they are seeing around them? Young children imitate, but they also show an innate sense of morality and justice.
Another point. Aggression is also taught and learned. Remember the baboons that lost all their aggressive males through food poisoning, and then the younger males were not conditioned to aggression and therefore were not aggressive? Life became very peaceful for that group of baboons after that.
Also, as Canard said, without the capacity for empathy and kindness in our mammal nature, we would have no concept of empathy and kindness to teach in the first place. We don't make up our nature. Life and evolution do. Our nature informs our concepts, not the other way around.
Humans have the capacity for a wide array of behaviors based in instinctive animal brain urges. Human cultures do reinforce some behaviors and suppress others and tendencies vary, but those animal urges are still part of our nature, and that mammalian urge toward closeness to other humans is just as basic as aggression.
In fact, I'd argue it's even more basic and essential. Aggression is not even needed until conflict or threats arise, but the drive to closeness is enjoyed all the time, as comforts in stressful times and just ordinary joys for the sheer pleasure of it in peaceful times.