mercilessness vs inhumanity

mercilessness and inhumanity both are nouns.

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

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

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

Other hypernyms of mercilessness include inhumaneness.

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