That's right, you don't. You also don't know if it might be the neighbor's 20 year old and a friend trying to get the key in the lock either.
You can't just start shooting until there IS a threat and one that is life threatening to someone.
In the above scenario I can see myself calling 911, taking an advantageous position and from there confronting the people in question and holding them until the police arrive.
I cannot see myself opening fire without attempting to verify the direct threat to someone's life. I would not, for example, raise the window and start shooting which some in here seem to advocate.
You can't hold them if they just takeoff running. You'd need a taser, which is not a substitute for a gun. Trying to subdue and restrain someone is possible, 1 on 1, provided you have sufficient training... but you don't know if they're armed or not. Fighting 1 on 2 is out of the question and a recipe for disaster.
Now here's where the fun part of Texas law comes in. I'll use my recent example. A guy plowed into my 4Runner and was quick to run off down the street...
Option 1: Jumping on his back and choking him unconscious. Rear naked choke = lateral vascular restraint, cuts off the blood flow, you go out in a few seconds.
Option 2: Taking the guy to the ground, mounting, dropping fists and elbows, and switching to an armbar. Armbars dislocate elbows and tear tendons.
Option 3: Pulling out my pistol and shooting him.
Option 4: Stand there like an idiot and hope the cops show up quickly.
Option 1, legal. Cops will let you skip and dance away, but it's a gamble because you don't know wtf he may be armed with. I'm told getting stabbed sucks pretty bad.
Option 2, illegal. You go to jail with the guy and get strip-searched.
Option 3, illegal. You go to jail, guy goes to morgue, you get strip-searched, Quannel X protests in your neighborhood.
Option 4, legal. However, I've had response times between 1 minute and 3 hours. It's a roll of the dice.
Now, option 3 becomes perfectly legal if the guy does something as simple as turning around and reaching into his pocket. You don't know what he has. The only safe option is to open up on him.
Now, back to the original situation, looking from a criminals perspective... I'm gonna run from you. I'm not gonna stick around and be arrested. On top of that, I know you have guns. I know everybody leaves their home at some point. I know exactly who to rob soon, because guns sell very, very well.