The Collaborative International Dictionary of English v.0.48:

Heartless \Heart"less\, a.
   1. Without a heart.
      [1913 Webster]

            You have left me heartless; mine is in your bosom.
                                                  --J. Webster.
      [1913 Webster]

   2. Destitute of courage; spiritless; despondent.
      [1913 Webster]

            Heartless they fought, and quitted soon their
            ground.                               --Dryden.
      [1913 Webster]

            Heartless and melancholy.             --W. Irwing.
      [1913 Webster]

   3. Destitute of feeling or affection; unsympathetic; cruel.
      "The heartless parasites." --Byron. -- Heart"less*ly,
      adv. -- Heart"less*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

heartlessly
    adv 1: in a heartless manner; "she behaves rather heartlessly
           toward her admirers"

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "heartlessly":
   apathetically, chickenheartedly, cold-bloodedly, coldheartedly,
   coldly, cravenly, cruelly, deadpan, dejectedly, despairingly,
   despondently, discouragedly, disheartenedly, disinterestedly,
   dispassionately, dispiritedly, downheartedly, dully,
   faintheartedly, forlornly, harshly, impassively, in cold blood,
   inclemently, inconsolably, indifferently, inexorably,
   like a coward, listlessly, mercilessly, numbly, phlegmatically,
   pitilessly, plucklessly, poltroonishly, relentlessly,
   remorselessly, ruthlessly, spiritlessly, spunklessly,
   uncompassionately, unconcernedly, uncourageously, undaringly,
   unemotionally, unfeelingly, unforgivingly, ungallantly,
   uninterestedly, unintrepidly, unmercifully, unpassionately,
   unremorsefully, unsympathetically, unvaliantly, unvalorously,
   unyieldingly, weakheartedly, with dry eyes