ruthful

O that my death would stay these ruthful deeds!

Gunnar let out a ruthful breath.

Complain not to the creature, this be 'plaint From one most Ruthful to the ruthlessest."

But you are also very funny and strippingly caustic and not too ruthful about what seems contemptible to you, or tiresome.

Allan M. Brandt's "Cigarette Century," a fat chronicle of the rise and fall of the cigarette in the 20th century, delivers that same ruthful stare.