We see the word "Ruthless" used (mostly to mean merciless or cruel, like a tyrant) but we rarely see "Ruthful".
Also, it's interesting that thesauri mainly list Sympathetic and Compassionate as antonyms of Ruthless. Likewise, Ruthless is generally not listed as the main antonym of Ruthful.
Guess it's just one of those things where usage sense and frequency has drifted w time. Nothing to rue though. 😉
The Spiral Jetty, a large earthwork sculpture made by Robert Smithson in 1970. It is constructed of basalt rocks and extends into the northern end of the Great Salt Lake. Human for scale, if you can find them (near the center of the spiral).