You have no idea what they would have done had he yelled, they could have rushed him and choked him to death, or ran and got a knife and came back and slit his throat and cut his eyes out or they could have fired their own weapons at him and blown his face off!
Whether they had a gun, knive or very sharp pencil is not the point. The point is they did not belong there, and the homeowner has to prepare based on the assumption that they are armed, not that they are not.
Just because you like to play armchair jury by looking back with hindsight and give criminals every benefit does not mean the homeowner was wrong in shooting intruders.
Just because you have a hard-on for living in the Wild West doesn't make it right. A homeowner has a right to defend himself and his property, yes. This guy went well beyond that. He lit a cigarette with a flamethrower. If he had yelled they probably would have taken off. He had his gun drawn and had taken aim, so if they kept coming he was ready. He didn't even try anything short of killing them. He's an ahole and a murderer and it takes a small, small mind to not see that.