I'm not really disagreeing with your main point, just the "it's obvious that would prevent death by a madman going on a shooting spree".
Hell, if the US banned guns I would still own one I would just hide it better.
You probably wouldn't, because the risk of ending up in jail or prison would be unacceptable.
However, as an anecdote, most people I know of who actually know what they are doing (as in people who are gun enthusiasts, not people who bought a gun "for protection" and shot it enough to take a class then put it in a drawer) announce themselves before shooting as the goal is to get the intruder to leave.
Quite often this alerts the intruder to your location and, in the case that they are an actual threat, they will charge at you (this is counter intuitive for me, but most stories I've read of home invasion with an armed resident seem to end this way). I only typically hear about these stories from survivors, but the cases I know of the resident killed or wounded their attackers and were safe.
Basically, that long-winded group of anecdotes is to say that it is not the case that a gun can only help you if you shoot first at any sign of home invasion. Anyone who actually knows something about guns knows that you only point a gun at a target you have identified and are prepared to destroy. For the most part, that means you won't shoot first at any sign of home invasion.
http://www.theonion.com/article/proposed-bill-would-bring-40...