The Collaborative International Dictionary of English v.0.48:

Cruelness \Cru"el*ness\, n.
   Cruelty. [Obs.] --Spenser.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cruelness
    n 1: the quality of being cruel and causing tension or annoyance
         [syn: cruelty, cruelness, harshness]