How are organised groups that consider themselves above the law formed?

Apparently the Crips gang was started as a neighbourhood watch against criminals. I'm interested in how violent groups are formed - paramilitaries and such. Anywhere where I can read more on the topic? A wikipedia article for instance. It's fascinating how someone can take a non-violent group and make it violent - I suppose there is research on the tactics - scapegoating and what not. From every field from how Jihadis are radicalised, to socialists and turned into passionate revolutionaries.