An evening in the park

There is a hired killer in the city park with a hitlist. There's a sheriff chasing the killer. A murder mystery type of game, where the killer has a hit list of 10 pictures to find and kill in the city park. 30+ NPCs accompany the players to make the park crowded. NPCs are jogging, walking, dancing and sitting around. Some of them are on the hit list and will be killed. While the killer can only be a player, NPCs can be the sheriff, too. Therefore the game can be played by one player only, NPCs can pick the dead sheriff's gun from the ground and become the new sheriff..