Die and you'll appear in a hospital, near its entrance. Head outside and you should find a cop waiting. Wrestle with him to steal his gun, then run back inside the building and hide behind the file cabinet near the entrance. Cops will enter one at a time now, looking for the troublemaker (that would be you), and you can shoot them and grab their ammo. Repeat this for long enough and it's easy to build up enough kills for both the Killstreak and Rogue Cop rewards.