It is the bad people who need to have principles to restrain them. For one thing, he liked to hurt people. It was for the strange reason that he was cruel, that the poor fellow never killed a man who asked for mercy, or committed a cruel action which he could have prevented.
T.H. White, The Ill-Made Knight
These Passions relate to a character’s choices with regards to vengeance and cruelty.
A Merciful character forgives harm done to them, and is lenient to those they have the advantage over. They may be called gentle, kind or benevolent - but their pity may be scorned as soft or even cowardly.
In contrast, a Ruthless character remembers slights and takes advantage when they can. They may be called stern, bloodthirsty or hard-hearted. When it is focused on returning evil for evil, they may be called vengeful - while those who act ruthlessly even without slight are often called cruel or vicious.