George S. Ledyard
Only about 1% of the popluation has any interest in martial arts training. Of those folks only a small percentage stays in long enough to acquire any real skills. So, statistically, the chances of a predatory type running into a trained martial artist is very small. On some level, I find the stories about the martial artist who succesfully avoids the conflict and walks away unsatisfying.
The one person the predator encounters who actually has the skills to handle that predator and he walks away.
I tend to agree that the skills of a martial artist put him/her under a moral obligation to protect his/her fellow citizens in (potentially) violent situations.
It is virtually guarenteed that this same predator will go off and find another victim, one who has little or no ability to defend himself or herself.
If the martial artist succesfully intervenes and stops the predator before really bad things happen, it may leave insufficient cause for prosecution and long term imprisonment of the offender. So unless the martial artist physically damages the predator to a degree that will prevent him from finding other victims ever again, intervening may not make much of a difference for future victims (like your story of the crazy neo-nazi).
Extending the responsibility of the martial artist from (potential) victims right now to potential victims in the future places a heavy burden on the martial artist. My feeling is that a martial artist should not be held responsible for this, because it's largely out of his/her control.