ruthless: fiercely violent and terror-inspiring, overpowering others by sheer force
This word paints a person or power that overwhelms others by brute force and leaves them trembling. It is used almost entirely from the victim's side, naming the violent oppressor who preys on the weak: the tyrant, the hostile army, the bully who takes what he wants because no one can stop him. In the prophets it scales up to whole empires, the fearsome nations whose armies terrorize their neighbors.
It grows out of a verb meaning "to dread" or "to strike with terror" (עָרַץ), and that terror is the heart of it. The person is not merely strong; the person is frightening. This is why it so often stands beside words for violence and evil yet is not the same as them: in Psalm 37:35 the writer sees "a wicked, ruthless man," where "wicked" (רָשָׁע) names the moral guilt and this word names the terrifying force behind it.
Because the word describes dread-inspiring might rather than evil in itself, it can even be turned to good. In Jeremiah 20:11 the prophet says the LORD stands with him "like a dread warrior," taking the very quality that makes oppressors so feared and placing it on the side of the one they persecute.