The Collaborative International Dictionary of English v.0.48:

Inhumanity \In`hu*man"i*ty\, n.; pl. Inhumanities. [L.
   inhumanitas: cf. F. inhumanit['e].]
   The quality or state of being inhuman or inhumane; cruelty;
   barbarity.
   [1913 Webster]

         Man's inhumanity to man
         Makes countless thousands mourn.         --Burns.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inhumanity
    n 1: the quality of lacking compassion or consideration for
         others [syn: inhumaneness, inhumanity] [ant:
         humaneness]
    2: an act of atrocious cruelty [syn: atrocity, inhumanity]

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "inhumanity":
   acuteness, animality, atrociousness, atrocity, barbarity,
   barbarousness, beastliness, bestiality, bloodiness, bloodlust,
   bloodthirst, bloodthirstiness, bloody-mindedness, brutality,
   brutalness, brutishness, cannibalism, cruelness, cruelty,
   destructiveness, extremity, ferociousness, ferocity, fiendishness,
   fierceness, force, furiousness, harshness, impetuosity, inclemency,
   inhumaneness, intensity, malignity, mercilessness, mindlessness,
   murderousness, pitilessness, rigor, roughness, ruthlessness,
   sadism, sadistic cruelty, sanguineousness, savagery, severity,
   sharpness, terrorism, truculence, ungentleness, vandalism,
   vehemence, venom, viciousness, violence, virulence,
   wanton cruelty