Ruthless
Part of speech: adjective
Definition: Without pity or compassion; cruel, pitiless.
Example sentence: The Asian woman in Hollywood movies has usually been one of two extremes - totally submissive or totally ruthless. In either case her primary function has been decorative.
Merciless
Part of speech: adjective
Definition: showing no mercy; cruel and pitiless
Example sentence: I used to have quite long hair, and I decided that I wanted to get it cut. I'd never met the person who did it, and she cut it into some kind of dreadful mullet. It looked like a triangle on my head. The other kids were merciless.