The Collaborative International Dictionary of English v.0.48:

Evilness \E"vil*ness\, n.
   The condition or quality of being evil; badness; viciousness;
   malignity; vileness; as, evilness of heart; the evilness of
   sin.
   [1913 Webster]

WordNet (r) 3.0 (2006):

evilness
    n 1: the quality of being morally wrong in principle or
         practice; "attempts to explain the origin of evil in the
         world" [syn: evil, evilness] [ant: good, goodness]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "evilness":
   arrantness, badness, baseness, damnability, devilishness, devilry,
   deviltry, fiendishness, flagitiousness, foulness, heinousness,
   hellishness, improperness, inaccuracy, inauspiciousness,
   inexpedience, inferiority, iniquitousness, invalidity, malevolence,
   moral badness, naughtiness, nefariousness, peccancy, rankness,
   reprehensibility, sinfulness, unfavorableness, unhealthiness,
   unkindness, unpleasantness, unskillfulness, viciousness, vileness,
   villainousness, wickedness