The best replacement for killer is nibbles.

The best alternatives to killer are rated as following:
★★★★★  nibbles
★★★★  beenie man
★★★★  programmer
★★★  computer programmer
★★★  vybz kartel
★★  murderer
★★  slenderman
★★  mass murderer
★★  coward
★★  computer geek
★★  zoro
★★  killer
★  jane the killer
★  diver
★  scientist
★  urouge
★  merciless
★  sizzla
★  computer scientist
★  musician
★  computer
★  jeff the killer
  slenderman rap battle
  broadcom
  hard boiled
  lawbringer
  mass murder
  kartel
  confirmed bachelor
  slender
  hipster
  geek
  killer cop

Your alternative for killer is not listed? Suggest!
Speaking about killer, one probably shouldn't forget about fertilizer.

Other search terms related to killer and their ratings are:
★★★★★ fertilizer
★ trimec
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Google Trends

Scale is based on the average worldwide traffic of killer in all years. Click on graph for more.
killer
nibbles
beenie man