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?
We easily give in to the emotions of hate and anger to other people because it is in the nature of the people. Genetics? Can be a probable cause of it, but we cannot deny the fact that it is already imprinted within our genes. Also put into consideration that the actions of other people also triggers hate. If one person looks "good" to you, sure enough you won't feel any hate towards him/her. But if he/she doesn't, you might start feeling some hatred towards him/her. Sometimes, it's also a matter of how you put things into perspective; the more you incline in one side, the less you will feel towards the other.