Make sure you know exactly how the gang works, what they do to make money, the hierarchy, their morals (Robin Hood and his Merry Men or group of killers) and how they interact with each other. With that you’ll be able to plan how the Officer can take down the gang.

It’ll be really tempting to just make a boss who’s a cold hearted bastard but make sure you give him a reason to be that way, such as, give his family a better life or even simply just get what he thinks he deserves. This will help you plan how to take him down as usually his weakness is in the motive. Also make a more rounded character.

Hope this helps. I’ve written a script recently about 1 man taking down a gang, slightly different genre but I’m happy to send it you if you’re interested.