To easily get "The Butcher" Achievement, which requires you to kill 50 people in five seconds (including pedestrians, military, infected etc.), wait until you have the ability to hijack a tank. Drive into a red zone that typically has a lot of fleeing normal and infected civilians and stick to driving on the side walk. Keep up with this same tactic to unlock Speed Bumps, which requires you to run over 500 people in a single tank.