Costumed human killers
Several posters mentioned the effective use of a miner's uniform complete with helmet and breathing apparatus to create a terrifying, human, yet non-human like crazed killer. I agree. This concept was also effectively employed in a horror slasher flick of the same year 1981, THE PROWLER. That movie's title was misleading. The killer in THE PROWLER was another deranged, psychotic man. His costume was equally scary. He dressed completely head to toe in World War II American soldier combat fatigues, as if he were just off the Normandy beaches. He covered his face with a WWII gas mask.
Other past Hollywood fright films occasionally used a masked killer but MY BLOODY VALENTINE and THE PROWLER killers were among the most creative for the use of full-body costumes based on real-world clothing. Hollywood hasn't gone for this idea much anymore. Today's killers don't wear masks and costumes in horror films like SAW and HOSTEL.