The number one contributing factor is the simple fact that it's just human nature to simply not like those who are different. Sure, as a species and a society we are getting better as time goes on at being tolerant and accepting, but as long as we continue to allow ourselves to seperate each other, the hate will not stop. After all, most groups and organizations are nothing more than a sugar coated mild form of hate. I understand seperation in sport simply because women and men will never be competitive equals, but all these, women only, black only, men only, white only clubs and groups? It's this constant bickering over petty crap that keeps all of us from prospering.
Depression. People who are depressed or find that they are have problems in their life are very vulnerable and they usually join groups who give them comfort. Its just sad that the group they join bring the same depression and hard life on others. Their are many groups in our lives and I guess the one that makes them feel most wanted are the hate groups because they are able to vent and hate without actually facing their real issues and work hard on sorting it out. Although they bring sorrow to people it is them I feel really sorry for.
I would say the biggest culprit is bias, and confirmation bias. Bias because this is generally what gives people the idea and justification for what they believe in, which in most cases is absurd, and confirmation bias is things that a person finds to further prove their point.
Confirmation Bias can be found here - http://funadvice.com/r/14s8rna7jo3
It's really a systematic cycle that's tedious and exhausting to break.
Ignorance. Poor education. Tradition. Peer pressure. Religion. Economic downfalls. Social statuses. Hate. Revenge. Their upbringing. The list goes on. Theres a million different reasons why people join hate groups. None of them make it right or justify it though.
because hate is always in everyone of us. Only love group do not let hate to take control while some do.