I'm going to diverge a bit from the majority and write plainly that some systems are more ruthless than others. While it is true that many systems have the potential for ruthless actions, to say Aikido, a primarily defensive style called the gentle martial art, is not as ruthless as say, American Combato, a very offensive form would not be true. That said, while I have the most experience with American Combato (Although only a beginner, never even tested for a belt) it was clearly taught that the eye gouges, kicks to the knees, strikes to the windpipe and the like that are taught in the system are only to be used in self defense and only deadly techniques used when your life is in danger and never use any of it unless you can't run. So, we were taught to basically be ruthless if there was no other option, do what you need to do to survive the attack, ruthless if needed.
I do have a yellow belt in Krav Maga (This means very little, after eight weeks we were all tested and we all passed. If you got 70% of a technique right you passed, it was a joke, not stating all Krav Maga classes are like this, merely the one I took) and I would again say it was more ruthless than a system like Aikido, but again the emphasis on only using it in self defense.
Personally, I would like it if more gentle systems like Aikido could indeed work against an attacker. I've been convinced by experts that they can't work, they may be wrong, but they convinced me. I think there are times you need to be ruthless to survive, and some systems are more ruthless than others, certainly regarding the damage done to the opponent. Than again, I'm no expert. And I'm sure you could kill even using Aikido, throw the guy on the head on hard concrete...he would likely die.