The Collaborative International Dictionary of English v.0.48:

Unmerciful \Un*mer"ci*ful\, a.
   Not merciful; indisposed to mercy or grace; cruel; inhuman;
   merciless; unkind. -- Un*mer"ci*ful*ly, adv. --
   Un*mer"ci*ful*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unmerciful
    adj 1: having or showing no mercy; "the merciless enemy"; "a
           merciless critic"; "gave him a merciless beating" [syn:
           merciless, unmerciful] [ant: merciful]

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "unmerciful":
   barbarous, bowelless, brutal, brutish, callous, calloused, cold,
   cold of heart, coldblooded, coldhearted, cruel, dog-eat-dog,
   flinthearted, flinty, hard, hard of heart, hardened, hardhearted,
   harsh, heartless, inclement, inexorable, insensitive, mean,
   merciless, obdurate, pitiless, relentless, remorseless, ruthless,
   savage, stonyhearted, uncompassionate, uncompassioned, unfeeling,
   unforgiving, unkind, unnatural, unpitiful, unpitying, unremorseful,
   unresponsive, unsparing, unsympathetic, unsympathizing, unyielding,
   vicious, without mercy