When we see someone who doesn't look like us, doesn't sound like us, doesn't act like us, doesn't love like us, doesn't live like us, the first thought we have about him is rooted in either fear, or judgement, or both at the same time. This is evolution. We have survived as a species by being suspicious of things we do not know.
If we are able to turn off this animal instinct and force the brain to follow a different path, then we gradually become kinder, more patient, and gain the ability to accept people, facts and knowledge that we are not used to. Empathy and sympathy are signs not only of some kind of philosophical and spiritual development, but also of physiological development. They require the mental faculties to overcome our most primal impulses.
When people show cruelty, it means that they have failed the first test of a developed society. They hardly forced their animal brains to develop beyond its first instinct. They almost did not create new neural connections, mental pathways to overcome their own instinctive fears. Therefore, their thinking and problem solving will lack the imagination and creativity that kinder people possess.
Over the past few years, I have come to realize that one thing is more or less a universal truth: the kindest person is often the smartest one. Just take a look at Dumbledore.