What if that layman isn't trying to be a hero. What if they are just exercising their right to protect themselves or their family?

When the police in a municipality have the monopoly right to posess and use force to protect citizens, then citizens should expect them to actually keep them safe. If they fail in this duty, citizens should have recourse.

Oakland in particular has a very difficult problem prosecuting criminals. The "stop snitching" culture that is prevalent in urban, black communities, combined with historical mistrust of police, translates to no cooperation and no arrests. Criminals aren't being deterred, and are acting with impunity. Probability is on b your side in keeping you safe, until one day you're the unlucky target. You won't be able to defend yourself, the other riders will not help and will pretend to not see you, and you will feel the helplessness that I once felt when I was savagely beaten by several laughing teens in DC next to a metro station. None of them were caught, but the police were sure helpful in telling me that the station I went to was a bad place for white people to go and I shouldn't go there anymore.

Layman with weapons isn't ideal, but neither is the delusion that the police can keep you safe in an area rife with cultural dysfunction and the young, borderline psyco men who target anyone that looks like they are productive for predation.

I know this sounds crazy, but I'd be willing to tolerate a modest number of bystander deaths if it meant a dramatic uptick in the number of criminal deaths. There can't be that many criminals. The steady-state equilibrium might be safer than the current regime, which is that criminals can assault (and kill) with little risk to themselves.

Of course I'd like my chance of death-by-bullet to be zero. But for whatever reason, that doesn't seem to be an option on the table.