You must kill 10 overtly friendly machines to earn this trophy. The neutral machines wandering around the City Ruins don't count as friendlies, so your first real opportunity to earn this will be in the Amusement Park.
If you kill 10 machines in the entry plaza to the park on your way in, you don't even have to feel bad about it - they'll all respawn when you make your way back out after defeating the boss of the area.