Easy way to boost this if you didn’t get it done during your story play through:
•Load up chapter 5
•Right at the beginning, head out to the street and you’ll see 2 civilians standing there.
•Take out your pistols and they’ll crouch down in fear, just walk up close to them and pull the trigger.
•Pause, checkpoint restart, rinse and repeat
*sometimes a police officer will spawn behind you down the street, giving you an extra kill
Can also be done to boost Darkness skill kills for the Darkness Master achievement