People tend to act irrationally when they believe they are in danger. Them having a gun on them can have seriously horrific consequences.

Of course, there are a lot of variables in each situation. The main question being whether, when a person is in danger, is it likely to be a "them or me" situation? Generally not, though it can seem like that to the person at the time.

On the police comment, I don't think so many really are comfortable with certain police units being armed to the teeth with weapons, considering how some have shown themselves to be as incompetent and irrational as a member of the general public. However, it generally is only a very small amount of specialised units who have access to them.

I could point out that people will act just as irrationally with knives actually, but I think I'll leave it as it is; that's a pretty common view on the pro-gun control stance.