The sad truth is that this agent is going to be fighting an uphill battle to try and prove that everyone he shot was endangering his life. If the crooks were in their car his vehicle was no longer in danger and you’re dealing with a property crime. He’s going to have to prove that he is proficient with the handgun he used and will probably get sued for shooting anyone other than the driver that was threatening his life with the vehicle.
He doesn't need to prove it. He was acting as a private citizen, not as a federal border agent. In Texas you don't need to be in fear for your life, you can use deadly force to defend property. As an added bonus, this was during the nighttime, when you can use deadly force to defend against simple theft and criminal mischief, as well as the other property crimes for which deadly force is allowed all the time, burglary, arson, robbery and aggravated robbery. SECTION 9.41. PROTECTION OF ONE'S OWN PROPERTY