People want a scapegoat
When you are struggling, whether it’s problems at work, low self-esteem, conflicts in your relationships, etc., it feels much better to funnel your negative energy into blaming someone else than to confront your own role in your problems. A lot of people join hate groups because it allows them to funnel the blame for all of their problems into another group of people while being supported by a group of people who share their beliefs and make them feel like they belong.
They’re lonely and seeking connections, even hateful ones
Many other people join hate groups because it fills their need for friendship and belonging. You don’t need to do or be anything special, all you have to do is be negative towards other people. It feels easy. Likewise, some people find it easier to make connections by putting others down and seeing who agrees than to prove to people that they are interesting and valuable companions.