"Marcille is blaspheming the Almighty Christian God with her evil Black Magic!" there is zero mention of any gods in Dungeon Meshi, everyone who expresses shock at her actions never brings up religion as a reason for why her actions are unethical.
Marcille is a mad scientist committing medical malpractice in a desperate attempt to save her lover. She's grafting dragon meat onto Falin's skeleton which turned her into an unbalanced chimera hybrid with a mouth too small to feed its body. She rapid-heals her friends without anesthetic and later admits healing magic has been used in torture techniques.
Marcille is a mad scientist. She's a gay Dr. Frankenstein. She's like if Gregory House were Italian.