[Brian Pfleuger]

I don't think that's true. While extensive training certainly increases your odds, most defensive situations are successfully resolved by folks with no training whatsoever.

Training would certainly be better but I don't think it comes down to required.

I suspect that the high likelihood of encountering armed resistance would stop most of these lunatic cowards before they ever acted. Especially if they didn't know who might be armed or when. They might just go somewhere else instead, but that's another issue altogether.

The presence of armed resistance may well NOT stop someone who has a specific grudge but someone who's looking for an easy target will likely look elsewhere.