You cannot however, use deadly force on an arsonist who has already started the fire....go figure.
A few years back, a guy got his hubcaps stolen at his apartment complex. He bought new ones, but put an alarm on his car. Someone set off the alarm and he saw a car racing away(at night) and his hubcaps missing. He gave chase, caught up with the car and Killed everyone in the car. He was no billed! The hubcaps were in the car.
Now, nothing about this incident suggests that deadly force was appropriate, even in Texas. But, an understanding of case law is necessary to determine if deadly force is allowed. Something you learn in conceiled carry classes.
That's going to cost you some day. Maybe more than you can bear. There is a vast difference between answering the door with a gun held discretely behind your back, or still in its holster and your hand on the butt, and coming out shooting.
I once opened my door, as I turned on the floodlights, to find a cop in my fenced back yard! Turns out he was responding to a strange noise complaint. The noise was me knocking my garage door back into alingement, but it was pretty late when I got home with the tool for the job. The cop never saw the gun, we exchanged explanations, and that was that.