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