The Collaborative International Dictionary of English v.0.48:

Remorseless \Re*morse"less\, a.
   Being without remorse; having no pity; hence, destitute of
   sensibility; cruel; insensible to distress; merciless.
   "Remorseless adversaries." --South. "With remorseless
   cruelty." --Milton.
   [1913 Webster]

   Syn: Unpitying; pitiless; relentless; unrelenting;
        implacable; merciless; unmerciful; savage; cruel.
        [1913 Webster] -- Re*morse"less*ly, adv. --
        Re*morse"less*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

remorseless
    adj 1: without mercy or pity; "an act of ruthless ferocity"; "a
           monster of remorseless cruelty" [syn: pitiless,
           remorseless, ruthless, unpitying]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "remorseless":
   bowelless, callous, cruel, dog-eat-dog, flinty, griefless, hard,
   harsh, heartless, impenitent, implacable, inclement, inexorable,
   merciless, pitiless, regretless, relentless, ruthless, savage,
   shameless, sorrowless, unashamed, uncompassionate, uncompassioned,
   uncontrite, unfeeling, unforgiving, ungrieving, unmerciful,
   unpitiful, unpitying, unregretful, unregretting, unrelenting,
   unremitting, unremorseful, unrepentant, unrepining, unrueful,
   unsorrowful, unsorrowing, unsorry, unstoppable, unsympathetic,
   unsympathizing, unyielding, without mercy