This one is very easy to get. Just take your Paladin or Ranger main character and start attacking random NPCs at any point in the game, and the achievement will unlock after a few kills. Might be slightly dependant on what sort of NPCs - for example, don't go for guards and adventurers.
Shouldn't take more than a couple of minutes, unless you have to start a new game. You might have to get out of the starting dungeon, though killing the three Dryads might be enough. (I have not tested that.)
Anyway, make sure to save beforehand, and reload after if you're not done with the Orderly Surroundings achievement, for which you'll also need a Paladin, or the Cabin In The Woods achievement, for which you'll also need a Ranger.