a person or thing that kills.

killer whale.

a device used by a post office for printing cancellations on postage stamps.

a mark of cancellation made on a postage stamp.

Slang . something or someone having a formidable impact, devastating effect, etc.

The math test was a real killer.

severe; powerful

a killer cold.

very difficult or demanding

a killer chess tournament.

highly effective; superior

a killer recipe for fried chicken.

noun

assassin , butcher , cut-throat , executioner , exterminator , gunman/woman , gunperson , hit-man/woman , hit person , hunter , slayer , soldier , cutthroat , homicide , manslayer , massacrer , murderess , slaughterer , triggerman , assassinator , destroyer , difficult , executor , gangster , gunman , hangman , hit man , murderer , sniper , tough