Why does hatred make man cruel towards his fellow man?
Hatred of others, whether for racial or social reasons, often serves as justification for cruel acts. Many people who harbor unreasonable bias or prejudice believe that their viewpoint on their targeted groups gives them the right to act in a cruel fashion, from simple taunting to outright violence. Simply put, hatred makes man cruel because it inherently allows cruelty as a necessary and normal part of everyday thought.
When hatred turns from personal beliefs to violence, it becomes the problem of society rather than family; no one has the right to force a view -- even a tolerant one -- on someone else, but cruel actions are the opposite side of that coin, since the cruel person has forced his view on others through speech or action.
Most come to hatred through their parents, who indoctrinate them from a young age to share their own warped views. Since the hatred is taken at face value as a simple truth, instead of arriving at the conclusion from a rational examination of evidence, people who grow up with hatred never think to question or reexamine their views. From that point, their actions, including cruelty, are motivated by a viewpoint that they consider completely rational, because they have never thought to question it. Cultural hostility towards other cultures is often akin to background noise; people with bias don't think about the bias because everyone around them agrees with it.
Examples of ingrained and inherited prejudicial groups include the New Black Panther Party and the Westboro Baptist Church.