Well thats your opinion and you entitled to it, except for the facts your trying to add. So tell me how you know this guy is unarmed? For all you know he is geeked up UFC fighter who could beat your ass like aint nothing to it. So get up on your high horse and ride out, cause I am not going to roll over for some guy who wants to run up in my yard and act crazy.
The problem with this is that it is YOUR responsibility to know the answer to your questions BEFORE you kill someone. By your logic, you should be able to shoot someone passing you on the sidewalk because you don't know he's not going to stab you as you pass him.