You're the one making assumptions here. I know/knew several people who've owned firearms, and in my, and the opinion of several others, they really shouldn't have been allowed to. 3 were alcoholics, 2 regular cocaine users (not all GPs will know each persons habits unless they've been for help), 1 bloke had been convicted of violently abusing his partner. 1 was a proper gun nut who would happily own loads of full automatic assault weapons, if he could. It was, apparently, relatively 'easy' to obtain licences. Point is, as you well know, there are lots of people out there who own firearms legally, that really, really shouldn't. And how easy is it, for someone with a licence, to also own unlicensed weapons? There's plenty of illegally owned guns out there, you know this.
I've twice had guns pointed at me by farmers, whilst legally on a RoW, shouting at me to get off their land. Good chance those were both 'legally' owned. Wouldn't have made much difference had they shot me though, would it?