“Ruth” was a common word in Middle English, first appearing (as “reuthe”) around the 12th century, meaning “pity or compassion,” and in the 13th century we spoke of a person who was kind, charitable, and just generally felt your pain as being “ruthful.” (“Ruthful” has also been used at times to mean “inspiring compassion or pity,” i.e., pathetic, as well as “expressing grief” as in “ruthful weeping,” but these are secondary senses.)
A person who lacked those qualities of kindness and charity, whose only concern was for personal gain and never shed a tear for the victims of his greed, has been, since the early 14th century, known as “ruthless,” literally lacking the quality of “ruth.”