merciless

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

Antonyms: giving kind merciful nice sparing sympathetic