It's possible you might have to actively hunt down more enemies to nab this achievement before finishing the game.
To kill a Human you MUST use fire, knocking them out doesn't count and they will need to be killed with fire, flaming bullets are easiest, or you can use a spray can with lighter, an explosive, or torched melee weapon (chair for example). Any and all kills will count, even if you die.
To find more enemies stay on the main roads, as they like to jump on your car, or you can head for the static circles on your map (Roots of Evil) as most of them have plenty of enemies nearby to kill.
Also a very quick way of doing this, go to the "underground complex" type building in the middle of the map by the lake, blow up the door with an explosive and enter the building, you will find 3 or 4 enemies inside (as well as tons of stuff to make explosives and such), simply kill them, exit the building, then reenter and the enemies will have respawned. Rinse and repeat until the cheevo pops.
That same building is also a great place to make combinations to help towards this annoying little cheevo: