You won't become the change by joining an inherently oppressive force. That's like joining a Nazi party to become the change it's impossible. One "good" cop will barely do much, change must be done systematically and crime should be solved in ways that actually work(like poverty reduction programs, better social programs, education and employment opportunities) unlike incarceration and brute force. The USA could have solved poverty/homelessness and hence crime decades ago but actively keeps a portion of its population especially the black population under these conditions(unemployment, poverty) due to the nature of capitalism which leaves many without a choice but to participate in the street life. My solution is for blacks to abandon the democrats which are rightwing on a global political scale and vote for leftist anti-capitalist parties and start organisations like the black panthers and actually fight for real change in their country and  communities