For me the argument for arming against criminals fell kinda flat with the shooting in new york last month... If I remember correctly the first headline was along the lines of "9 shot at empire state building... police kill shooter"...
then it turned out the shooter was not a gunman run amok, he had had killed one specific person... the 8 other folks shot were hit by 2 policemen out to save the folks around them...
imagine the total of people shot if just 2 or 3 civvie bystanders had pulled out guns... the cops would have shot them as well, not knowing they were "good guys"... unless they had shot each other first, thinking the other shooters in civvies clothing were probably bad guys as well.... just run the scenario through your head and figure how it would have played out...
A guy shoots another guy on the street... cops arrive and open fire hitting 8 people... 3 other guys pull guns and suddenly lead is flying and noone knows who is who...
Then another cop arrives... do you think he is going to assume that a civvie with a gun is a good guy? Will he take a chance?
Total clusterfck.