The Collaborative International Dictionary of English v.0.48:

Inhuman \In*hu"man\, a. [L. inhumanus: cf. F. inhumain. See
   In- not, and Human.]
   [1913 Webster]
   1. Destitute of the kindness and tenderness that belong to a
      human being; cruel; barbarous; savage; unfeeling; as, an
      inhuman person or people.
      [1913 Webster]

   2. Characterized by, or attended with, cruelty; as, an
      inhuman act or punishment.

   Syn: Cruel; unfeeling; pitiless; merciless; savage;
        barbarous; brutal; ferocious; ruthless; fiendish.
        [1913 Webster]

WordNet (r) 3.0 (2006):

inhuman
    adj 1: without compunction or human feeling; "in cold blood";
           "cold-blooded killing"; "insensate destruction" [syn:
           cold, cold-blooded, inhuman, insensate]
    2: belonging to or resembling something nonhuman; "something
       dark and inhuman in form"; "a babel of inhuman noises"

Moby Thesaurus II by Grady Ward, 1.0:

80 Moby Thesaurus words for "inhuman":
   Draconian, Tartarean, animal, anthropophagous, atrocious, barbaric,
   barbarous, beastly, bestial, bloodthirsty, bloody, bloody-minded,
   brutal, brutalized, brute, brutish, callous, cannibalistic,
   cold-blooded, cruel, cruel-hearted, demoniac, demoniacal, demonic,
   demonish, demonlike, devil-like, devilish, diabolic, diabolical,
   fell, feral, ferine, ferocious, fiendish, fiendlike, fierce,
   ghoulish, heartless, hellish, implacable, infernal, inhumane,
   insensitive, kill-crazy, malicious, malign, malignant, merciless,
   murderous, noncivilized, ogreish, pitiless, relentless, ruthless,
   sadistic, sanguinary, sanguineous, satanic, savage, severe,
   sharkish, slavering, subhuman, tameless, truculent, unchristian,
   uncivilized, uncompassionate, unfeeling, ungentle, unhuman, unkind,
   unkindly, unrelenting, unsympathetic, untamed, vicious, wild,
   wolfish