mercilessly vs pitilessly

mercilessly and pitilessly both are adverbs.

Word	Noun	Adjective	Verb	Adverb
mercilessly	No	No	No	Yes
pitilessly	No	No	No	Yes

As adverbs, mercilessly and pitilessly are synonyms defined as:

Other synonyms of mercilessly include remorselessly, unmercifully.

mercilessly (adverb)	pitilessly (adverb)
without pity; in a merciless manner	without pity; in a merciless manner