For this achievement just use any of the melee attacks to kill 50 residents.
Heres a list of which weapons count and the costs:
.Fists - Default and free
.Baseball bat - $10 - (i think this is best to pick as its moderately powerful and has a reach so not as close up as fists)
.Crowbar - $20 - most powerful melee weapon but its slow
. Knife - $10 - not very powerful and its close combat
.Nightstick - free - kill a cop to get this one, its moderately powerful.