mercilessness vs inhumaneness

mercilessness and inhumaneness both are nouns.

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

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

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

Other hypernyms of mercilessness include inhumanity.

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