Historically, 'militia' meant any member of the community who owned a firearm.
In modern terms, I contend it best means some sort of civil defense network of community members, preferably ones that actively participate in disaster planning as well as marksmanship.
As for #2 on your list... having *some* of the population with concealed carry raises the risk for *any* one planning a mugging/robbery. The result is actually usually an increase in petty theft or other crimes that don't require confrontation.
As for #1 .... well, that's a problem with any sort of tool that can kill, hammers, bats, gasoline, etc. But we could DEFINITELY use better integration of mental health data with the NCIS checking system. That would have flagged Holmes, for example.