unmercifulness vs inhumanity

unmercifulness and inhumanity both are nouns.

Word	Noun	Adjective	Verb	Adverb
unmercifulness	Yes	No	No	No
inhumanity	Yes	No	No	No

As nouns, inhumanity is a hypernym of unmercifulness; that is, inhumanity is a word with a broader meaning than unmercifulness:

unmercifulness: inhumaneness evidenced by an unwillingness to be kind or forgiving
inhumanity: the quality of lacking compassion or consideration for others

Other hypernyms of unmercifulness include inhumaneness.

unmercifulness (noun)	inhumanity (noun)
inhumaneness evidenced by an unwillingness to be kind or forgiving	an act of atrocious cruelty
	the quality of lacking compassion or consideration for others