mercilessness vs unmercifulness

mercilessness and unmercifulness both are nouns.

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

As nouns, mercilessness and unmercifulness are synonyms defined as:

mercilessness and unmercifulness: inhumaneness evidenced by an unwillingness to be kind or forgiving

mercilessness (noun)	unmercifulness (noun)
inhumaneness evidenced by an unwillingness to be kind or forgiving	inhumaneness evidenced by an unwillingness to be kind or forgiving
feelings of extreme heartlessness