cruelty vs cruelness

cruelty and cruelness both are nouns.

Word	Noun	Adjective	Verb	Adverb
cruelty	Yes	No	No	No
cruelness	Yes	No	No	No

As nouns, cruelty and cruelness are synonyms defined as:

cruelty and cruelness: the quality of being cruel and causing tension or annoyance

Other synonyms of cruelty include harshness.

cruelty (noun)	cruelness (noun)
a cruel act; a deliberate infliction of pain and suffering	the quality of being cruel and causing tension or annoyance
the quality of being cruel and causing tension or annoyance
feelings of extreme heartlessness