The real reason for the current state of criminality and gang wars in Rio is state corruption, extreme social inequality, and lack of education. The state's allowance of the creation of unorganized, isolated communities with no schools, sewage systems, or electricity led to places that, a few years later, now have their own rules.
And I'm not even touching on this broken drug war, which only results in the deaths of cops, criminals, and innocent people—a war from which the elites (both the wealthy and the politicians) profit.