The first chance you get to unlock this achievement is in Mission 2, it's a very short mission so if you somehow get through it without unlocking this achievement just boot it back up and try again!
All you have to do is run someone over, there are plenty of people standing in the road throughout Mission 2 and chances are you won't even have to try for this achievement because they're normally in the middle of the road and hard to miss!
However, I've linked in a video so you can see it being done, as stated in the video while the achievement doesn't pop up on screen that is how you unlock it, it's because he already unlocked this achievement on a prior playthrough of the level!
* Credit to savgcom for the video