Exactly thats the twist,

the gang Commits crimes daily but they cannot be arrested because the police has no evidence. and this batch of Not-so-good Cops are the only people who have the Guts to Defeat the criminals. 

We don't All have to be complete scumbags, Like for an example we can have a cop whos a Nice dude but is greedy. and a Angry cop who cares about his family a lot
the possibilities are [u]Endless![/u]

Also it shows that everyone has Flaws and Anyone can be a hero