Many times throughout history gangs form for self protection in absense of a functioning authority. Regardless they inevitably get involved in a racket of some sort or assert their claim to some sort of territory and become just another band of violent thugs out for themselves and a scourge on their host community.
The remotely closest thing to a gang that hasn't progressed to that probably isn't like that are the Guardian Angels safety patrols.
Being brutalized is neither neccessary nor sufficent, although it may make them more likely. If the "configuration" arises essentially they will go down that path.