Just to add to what ItsJust1n said, be sure to kill the 3 prisoners that you save for the Rescue Ranger achievement.
You MUST kill them before you "save" them or else they will become friendly and cannot be killed.
Thanks to Warboy925:
In order to kill those prisoners, you have to let the Nazi kill one first, then you kill him and the 2 remaining prisoners.
The achievement WILL pop just after killing the last guy, so if it hasn't popped yet, look around.