In Texas two things would have had to happen for that to be a legit shoot.
A) There has to be a "threat of imminent serious bodily injury or death", or
B) The vehicle would have to represent an irrecoverable or irreplaceable asset that would put an immediate and serious burden on the shooter's ability to provide for their livelihood. (Example: You are a mobile repair guy who works out of the service truck that contains all your parts, tools, and records. The truck IS your business. So someone stealing it would be, in essence, putting you out of business in an immediate and irrevocable basis.)
Since the perp was driving away the "threat of immediate serious bodily injury or death" did not exist. You didn't mention anything about the vehicle being other than an average everyday conveyance, so I will presume just that.
In many states leaving a car running with the keys in the ignition, even if locked, is a no-no. In Texas it's a misdemeanor offense. If your car happens to get stolen as a result, not only will they slap you with a citation, your insurance company will no-no the claim. (Check your auto policy. I bet it specifically talks about losses due to negligence, and how claims for such will be denied. In other words, if you create a condition that makes it reasonably easy to get in the car and drive away, you can say goodbye to any help from your insurer.) If a citation is issue to the shooter for not securing the vehicle, that will be a huge problem for his defense. The prosecution will use that to assert that the vehicle must not have been very important to the shooter or he would have taken more care in securing it. This goes directly to part B above.
Sounds like the shooter is in mucho hot water no matter how scummy the vehicle thief might have been. If the above scenario is even remotely accurate, his only real hope will be some legal technicality, one helluva good defense attorney, or jury nullification.
Brad