Complex sentence: Cass found through violence the respect he never had and became addicted to the buzz of fighting and become involved in football hooliganism via the Inter City Firm, whom he went on to become the de facto leader of the West-Ham Football Firm.
Split into simpler sentences: Cass found through violence the respect he never had and became addicted to the buzz of fighting. He became involved in football hooliganism via the Inter City Firm, for whom he went on to become the de facto leader of the West-Ham Football Firm.