Some synonyms include merciless, pitiless, cruel, unmerciful, remorseless, relentless, heartless, grim, unrelenting, uncharitable, and hard-hearted.
If you were to say someone is ruthless, it would be referring to their lack of pity or their harsh and cruel demeanor.
The word ruthless dates back to early 14th century. It comes from the late 12th century term reuth which refers to pity and compassion. The word reuth was formed from reuwen, "To rue" on the model of truth.
Ruthful
The word ruthful was used during the 13th century, but has since fallen out of modern use and is almost exclusively used as a deliberate archaism.
Sab. What doleful sight, what ruthful spectacle, Hath Fortune offered to my hapless heart ? My father slain with such a fatal sword ! —My mother murdered by a mortal wound !
In other words, the death of his mother and father is a pitiful and sorrowful event or sight.
'To rue on the model of truth'
The verb rue is used in place of sorrow, regret, or sadness associated with some wrong done or some disappointment.
