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Adjective: pitiless  pi-tee-lus
  1. Without mercy or pity
    "a monster of pitiless cruelty";
    - remorseless, ruthless, unpitying
     
  2. Deficient in humane and kindly feelings
    "The pitiless dictator showed no mercy to his opponents";
    - unkind

See also: inhumane, merciless, unmerciful