For clarification's sake: When I write "ruthless," I don't mean burning down towns and kicking elderly people.
Webster's defines "ruthless" as "having no ruth : MERCILESS, CRUEL."
When I use the term, I meaning having no mercy upon one's enemies and demonstrating cruelty to them, because they have chosen to oppose you. Cruelty not in the sense of killing their dog because you don't like them, but in the sense of an absolute void of humane feelings, since humane feelings are not necessarily conducive to one's survival.
For example, when you knock down your enemy, you kick him. When he sleeps, you hurl a grenade into his chambers. When he goes to the toilet, you strike. When he goes to confessional, you stab him with your dagger. (To steal an image from Hamlet.)
In other words, you observe your enemy, select the most vulnerable moment, and strike with the most deadly means at your disposal. You don't engage in soliloquy or quips. You don't posture or preen. You strike fast, strike first, and strike hard. Precise. No mercy. No surrender.
Ruthlessness is a cold, unswerving, unrelenting process of the systematic destruction of one's enemies.
Ruthlessness is proactive, not reactive. It seeks the enemy in its lair. It identifies and destroys the threat before it becomes a threat.
It's not random. It's not careless.
It is measured, planned, calculated, and executed.
Most people, at this point, experience a "Holy crap!" moment when the skin crawls and the stomach turns just a bit. That feeling you have is the feeling your enemies should feel.
That's ruthlessness.