He could have killed them on the spot. Legally.
Chapter 9 of the Texas Penal Code describes deadly force as justified to prevent arson, robbery, theft or criminal mischief at night, or to prevent a suspect from fleeing if the property owner "reasonably believes the land or property cannot be protected or recovered by any other means; or the use of force other than deadly force to protect or recover the land or property would expose the actor or another to a substantial risk of death or serious bodily injury."
"You hear someone stealing something off your front porch. You come out there with a gun, and they're running off. It's nighttime. The law in Texas allows you to shoot them," said former Dallas County prosecutor Toby Shook.