cruelty vs viciousness

cruelty and viciousness both are nouns.

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

As nouns, viciousness is a hyponym of cruelty; that is, viciousness is a word with a more specific, narrower meaning than cruelty:

Other hyponyms of cruelty include brutality, ferociousness, savagery, murderousness.

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