pitilessness vs unmercifulness

pitilessness and unmercifulness both are nouns.

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

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

pitilessness: mercilessness characterized by a lack of pity
unmercifulness: inhumaneness evidenced by an unwillingness to be kind or forgiving

Other hypernyms of pitilessness include mercilessness.

pitilessness (noun)	unmercifulness (noun)
mercilessness characterized by a lack of pity	inhumaneness evidenced by an unwillingness to be kind or forgiving
feelings of extreme heartlessness