The intruder was probably drunk, otherwise he would not have gone to the back yard.
A friend told me a story, how in his misspent youth, he became very drunk, and went to a stranger's house, thinking it was his own home, and the police were called.
Thankfully, my friend was not killed, as he matured, stopped drinking and today is an upstanding citizen.
I'm sure that this poor guy just took a wrong turn leaving the bar and ended up at this house he probably thought was his. Was he too drunk to forget his real ID? Since when did being drunk fly as an excuse? Why would a harmless drunk choose to fight a homeowner with a pistol? And why would a homeowner choose to shoot a harmless drunk?