for example, a gun is about 120% useless if you have it stored and the bad guy has the drop on you, which is likely the case on multiple levels given that that the 'bad guy' is the aggressor and initiator of a rare situation.
for example, the 'bad guy' may have a gun himself, and seeing you wielding one may cause him to shoot you, when he otherwise wouldn't have. and it's entirely possible or even likely that he has spent more time at the shooting range than you, or that he has killed people before. nevermind that he, having the mental preparedness that comes from being the initiator, may have the clearer presence of mind and less shaky hands
Etc. Etc. the "gun in my home to keep me safe" is a non-sequitur and only makes sense in simplistic cowboy scenarios, and I've even heard statistics saying you're more likely to hurt yourself or a loved one than the bad guy...
...but we don't really have any good ideas about that, because the gun lobby has suppressed research into these things for several years