Something I haven't seen mentioned here, but I think worth bringing up is that in many states with Castle laws, your "Castle" extends to your car. I don't know the gun laws, per se, in Texas (I have a cursory knowledge of them as I visit Texas once or twice a year) but in Arizona, the physical act of breaking into a home makes lethal force justifiable, with or without a weapon present. I am not saying you should shoot anyone who forces (meaning you've taken precautions to prevent entry, and they still force their way in) their way into your home (how many drunks, going to the wrong house have been shot?), but it is justifiable, since the person's intent has been shown.
If the OPs scenario happened in Arizona, showing the firearm would have been perfectly legal, as the OP was protecting his "Castle."
To those that say the first time an attacker sees your weapon it should be followed by loud bangs, keep in mind that most of the time, the mere presence of a firearm in the hands of an innocent, law abiding citizen is enough to deescalate most situations. That means everyone gets to leave the situation without any new holes in their bodies, and in good relative health.
Someone mentioned action beats reaction. This is very true when both attacker and victim are on equal footing. But if I have a gun in my hand, or my hand on my holstered gun, or whatever (no matter where it is), and I see an attacker making a furtive movement for his beltline, even though I am the one reacting, I will win this race every time. This is actually a fun drill to do with another person using a blue gun, or airsoft. Have one person with a (blue or airsoft) gun holstered. Have the other person with gun in hand. The only rule is, the person with gun in hand cannot aim and fire until the person with gun in holster makes a movement for his gun. I've done this one, and seen it done hundreds of times, and every time, with only a few exceptions the person with gun in hand wins.