this achievement is obviously easy
, just go around killing NPCs with any lethal weapon. fists DO NOT work. and most/all electric weapons will not work.
this is much easier to avoid! on your "Saint" playthrough.
avoid the headache and save it for a lethal run/playthrough.
Personally i got almost every other achievement on my saint playthrough, except these three, this one of course (this achievement directly conflicts with the much harder to get "Saint" achievement).
the 2 other achievements i missed included, Killing Ryan Andrews (shocking biology) which again i would NOT attempt on a "saint" playthrough.
to sum it up
: to avoid headaches, do a saint achievement run and then do a separate run for the kill 100 npcs achievement as well as killing Ryan Andrews (shocking biology achievement).