This is a wellknown story:
One evening an old Cherokee told his grandson about a battle that goes on inside people.
He said, "My son, the battle is between two wolves inside us all"
"It is a terrible fight and it is between two wolves. One is evil - he is anger, envy, sorrow, regret, greed, arrogance, self-pity, guilt, resentment, inferiority, lies, false pride, superiority, and ego." He continued, "The other is good - he is joy, peace, love, hope, serenity, humility, kindness, benevolence, empathy, generosity, truth, compassion, and faith. The same fight is going on inside you - and inside every other person, too."
The grandson thought about it for a minute and then asked his grandfather, "Which wolf will win?"
The old Cherokee simply replied, "The one you feed".
http://www.psychologymatters.asia/article/65/the-story-of-the-two-wolves-managing-your-thoughts-feelings-and-actions.html
But why do we so easily succumb to feed the evil wolf? Why do we hate and feed it and react hatefull towards others?
Sometimes we hate on others because it can make us feel better about ourselves. We think that because we dont have their flaws, we are better than them, when in reality we have flaws of our own and are not any better.