The Collaborative International Dictionary of English v.0.48:

Merciful \Mer"ci*ful\, a. [Mercy + -ful.]
   1. Full of mercy; having or exercising mercy; disposed to
      pity and spare offenders; unwilling to punish. Opposite of
      merciless.
      [1913 Webster]

            The Lord, the Lord God, merciful and gracious. --Ex.
                                                  xxxiv. 6.
      [1913 Webster]

            Be merciful, great duke, to men of mold. --Shak.
      [1913 Webster]

   2. Unwilling to give pain; compassionate.
      [1913 Webster]

            A merciful man will be merciful to his beast. --Old
                                                  Proverb.
      [1913 Webster]

   Syn: Compassionate; tender; humane; gracious; kind; mild;
        clement; benignant.
        [1913 Webster] -- Mer"ci*ful*ly, adv. --
        Mer"ci*ful*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

mercifulness
    n 1: the feeling that motivates compassion [syn: mercifulness,
         mercy]
    2: a disposition to be kind and forgiving; "in those days a wife
       had to depend on the mercifulness of her husband" [syn:
       mercifulness, mercy] [ant: mercilessness,
       unmercifulness]
    3: leniency and compassion shown toward offenders by a person or
       agency charged with administering justice; "he threw himself
       on the mercy of the court" [syn: clemency, mercifulness,
       mercy]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "mercifulness":
   acceptance, bleeding heart, bowels of compassion, clemency,
   clementness, compassion, compassionateness, easiness,
   easygoingness, forbearance, forbearing, gentleness, humaneness,
   humanity, laxness, lenience, leniency, lenientness, lenity, mercy,
   mildness, moderateness, patience, pity, ruthfulness,
   softheartedness, softness, tenderness, tolerance