People who live on farms or in the boonies have shotguns and handguns for several reasons, two of which are these:

1. To shoot wild animals who are killing their livestock and/or are threatening them.
2. Yes, to shoot humans who are threatening their families since it will be awhile before a police officer can get there. The response time in the boonies isn't like the urban or suburban areas.

The first, obviously, is more common, but both can be necessary.

On edit: Not trying to argue, but I think your last point should be the first point. Most of the people who go on these murderous sprees obviously have mental issues. I don't own a gun, myself, but both my husband and I are professionally trained to use them. We live in a city and have dogs. We choose not to own one because we also have children, but I can't say that everyone who owns a handgun is going to fly off the edge.