pitilessness vs mercilessness

pitilessness and mercilessness both are nouns.

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

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

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

Other hypernyms of pitilessness include unmercifulness.

As nouns, pitilessness and mercilessness are synonyms defined as:

Other synonyms of pitilessness include cruelty, ruthlessness.

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