2) Let me see if I'm following your hypo. You break onto my property carrying a bat. I point a gun at you. Is this accurate?
At that point, yes, you can probably conclude that I'm going to shoot you, and can react accordingly. If I shoot you and you're running away, I have committed murder. If you charge at me with the bat, I am, at that point, under threat of deadly force, and can, therefore, protect myself with deadly force and kill you.
The whole point is that you're not supposed to assume anything. You're supposed to react with appropriate force under the situation, and the law doesn't support pre-emptive killings. In this situation the killers didn't even know the guy was armed, and they certainly never saw him attempting to use any weapons. The law cannot and does not allow you to kill someone on the off chance that they might represent a threat. You have to wait until they actually indicate that they're attempting to use either deadly force or force that might cause great bodily injury. That didn't happen here. If you think about it a little bit this makes sense. Under your logic, it'd be legal to shoot anyone who unlawfully enters your property under the theory that they might be a threat, even if you have no reason to believe that other than the fact that they're on your property, and even if it turns out that they're totally unarmed and not remotely interested in engaging in any criminal activity. Shooting someone in such circumstances is incredibly reckless, dangerous, and generally a bad idea.