The Collaborative International Dictionary of English v.0.48:

Ruthless \Ruth"less\, a.
   Having no ruth; cruel; pitiless.
   [1913 Webster]

         Their rage the hostile bands restrain,
         All but the ruthless monarch of the main. --Pope.
   [1913 Webster] -- Ruth"less*ly, adv. -- Ruth"less*ness,
   n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ruthlessness
    n 1: mercilessness characterized by a lack of pity [syn:
         pitilessness, ruthlessness]
    2: feelings of extreme heartlessness [syn: cruelty,
       mercilessness, pitilessness, ruthlessness]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "ruthlessness":
   animality, atrociousness, barbarity, barbarousness, beastliness,
   bestiality, bloodiness, bloodlust, bloodthirst, bloodthirstiness,
   bloody-mindedness, brutality, brutalness, brutishness, cannibalism,
   cruelness, cruelty, ferociousness, ferocity, fiendishness,
   fierceness, flintiness, hardness, harshness, heartlessness,
   inclemency, inexorableness, inhumaneness, inhumanity,
   mercilessness, pitilessness, relentlessness, remorselessness,
   sadism, sadistic cruelty, sanguineousness, savagery, short shrift,
   tender mercies, truculence, uncompassionateness, unfeelingness,
   unforgivingness, unmercifulness, unremorsefulness,
   unsympatheticness, unyieldingness, vandalism, viciousness,
   violence, wanton cruelty