To get this achievement you will have to kill 30 criminals (10 criminals on each island).
You will have to steal a police car or just call 911 on your mobile/cell phone and the police will show up where you are (but they won't be looking for you), then access the police computer (you can only do it with the car totally stopped) and look for the "most wanted". A list of criminals will come up and start killing them one by one, the ones you need to do are in black, the ones u have killed are in red.
TIP: make sure you have a lot of money because you will need to buy ammo and body armor every 3 or 4 missions since the criminals won't be alone most of the time (they will have mates helping them out and you will be shot at a lot). Also note that you won't make any money out of the missions.