That's a nice, completely farcical worldview you've got there. Someone is either sane and responsible, law abiding, or they are not, and we will somehow be able to tell the difference between the two until the end of time. That sane person will never snap or change. They will also never ever allow their gun to be stolen. Nope, never.
This isn't about gun control nuts acting like anyone who carries a gun is itching to use it. It's about realism. It's about knowing that statistically, people can indeed go from totally sane (or just seeming that way from the outside vantage point) to terrible in what seems like the blink of an eye. The human psyche and humans in general are fragile creatures.
There are no "good people" and "bad people." We, in general, tend to swerve back and forth betwixt the two, some people swerving far, far more than others can imagine. If you are giving guns to who you deem to be responsible people, then there's still a statistical chance that some of them are not, and the more you hand out the more that statistic goes up. That's reality. The more you try and control for some of these circumstances and put in some sorts of failsafes, the more that statistic goes down. People who advocate for gun control aren't sitting here thinking all people with guns are maniacs. They're thinking that, statistically, some will be, and we don't want however few that is to exist... or at least we want that number to go down.