Merciless

cruel hard harsh heartless severe callous fierce grim mortal unfeeling ruthless implacable pitiless relentless unforgiving unmerciful unrelenting unsympathetic compassionless hardhearted inhumane unsparing unyielding barbarous inexorable wanton cutthroat iron-fisted dog-eat-dog gratuitous unpitying unflinching hatchet job mean machine having a killer instinct uncalled-for