Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

unmercifulness
     n : inhumaneness evidenced by an unwillingness to be kind or
         forgiving [syn: {mercilessness}] [ant: {mercifulness}]