Soldato
If someone is prepared to attempt to "invade" your property you must assume that they have a plan for what to do should they happen to meet you, I know I certainly would have a plan as to how to what to do if I meet the owner of wherever it is I'm invading.
Following from that, it is safe to assume that they may well assume that a typical house owner they meet may well have access to a knife, as such they would come with something that can trump a knife in a meeting situation.
To me the most obvious object to trump a knife is a gun of some kind.
And if you (yourself armed with some sort of pistol) come across the owner armed with a shotgun, you would first run for cover (so as to not get shot), and then attempt to shoot the owner so as to carry on with your intended activity (probably burglary)
Indeed..
What you must take into consideration is panic and adreanaline, the burglar having arrived at this situation is an amatuer, unless the homeowner is a ex-soilder, they are both a bit scared.
The urge to run is high, few seconds later your brain kicks in and you realise you have a gun and then shoot back, or you may just keep running..
Either way what was going to happen is irrelevent, what was going through the homeowners mind was fear, and fast response he saw the gun?? (there was no gun) but that doesnt matter it was all over in 2 seconds, its self defense because of what he was thinking..