There are several reasons why I generally disagree with this, but for the time being I'll go with the role of the penal system. You imply punishment, and that's at best a lesser role, IMO. Granted, it seems to be the only role people care about nowadays, as demonstrated by this thread, but by focusing solely on the punitive aspect you're actually creating the timidness people have about holding people accountable. Of course people are going to be trying to get at whether or not somebody was mentally competent when the result determines whether or not he's eventually destroyed as a human being.
I don't know that my response only involves "punishment". I'm okay with the other roles as well (rehabilitation, vengeance, and incentive, to name three), but the point is that personal accountability is inherent in ALL of them. You can't rehab someone that doesn't take responsibility for their actions. Incentive is lost if you push the idea that "we all have our excuses". Honestly, I don't care about the methodology; the idea for me is that whatever system you have, when you measure the start and the finish that you have less people committing the crimes at hand, less people being victimized by the crimes at hand, and less people interrupted from their daily lives by the crimes at hand.
Fair enough. In American justice when I hear personal accountability I assume vengeance. Sadly, that is the nature of things. However, even applied to rehabilitation and deterrence, I still think there need to be degrees of accountability. As with so many things, I think it's the conflict between simple and complex that set people apart. Accountability isn't binary, at least in my opinion, and a whole lot of people get pissed off when it isn't demonstrated as such.
In a causal chain you can find any number of links. In a perfect world you can address all of them. In this situation it seems you want to hold one person accountable for the entire effect because one link was his. The other links don't exist?
No, just the opposite; I want to hold ALL persons accountable. The guy with the badge doesn't get a pass because he has a badge, and the guy in the street doesn't get a pass because he's of color. "Tend Your Own Garden". I can't control you, I can only control myself. So if I account for all our interactions as if I am responsible, and you do the same (for the same reasons) we've greatly reduced our chances for conflict.
But there are links that exist beyond accountability to a specific case. One of my favorite examples is Henry Lee Lucas. We fault him for not breaking his link, but there were dozens of others well outside his control. We hold him accountable for not stopping something that myriad other factors put into play? On the basis that others might have?
And as to your greater end, I think that's a largely systemic problem, where our present mechanism fails quite hard. Personal accountability might be one of the problem points, but not a particularly large one in the grand scheme of things.