They are about the same. When you see a word that ends with the suffix "-less," it means "without." Therefore: Merciless = Mercy + less = "without mercy" (note that for spelling's sake the "y" in mercy changes to an "i") Mercy means to show compassion toward someone, despite having the power to hurt or punish someone. It may also be justifiable by some that someone should be punished. Ruthless = Ruth + less = "without ruth" Ruth is an old word that is no longer used on it's own. It means "pity." I felt I should breakdown each of the words to give you a detailed explanation of how they are the same and also so you will know how to take some words apart based on their prefixes and suffixes. Both of these words are synonyms of each other and can therefore be used interchangeably. If you have any other questions, please let me know.