Sigh...good guys with guns, again? Right. Your good guy with a gun is only that up until the moment he decides to go on a mass shooting spree. Is he still a good guy with a gun then?
That's why we should stop thinking that government employees are always good guys too. I've been through more background checks than most cops, and through about as much training as most young cops - but that doesn't make a difference in most gun control schemes that get discussed in the US.